Whis says, “By the time sensations pass through your five senses to your brain and are then transmitted back to your body, precious time has been lost. Ideally what you want is for each part of your body to react, respond and move of its own accord…”
Whis talks about sending signals to the brain and back to the body. To dodge a punch, your eyes must see it, the image of that punch has to be processed into your brain and a decision has to be made whether to block, dodge, parry, etc. A human that decides what action must be taken based on this information. All this time thinking and processing in the brain is time that is wasted. However, even if you reduce all this time to 0, there is also the time it takes for a signal to travel from the brain to the muscle.
Want to know how fast that actually is? The average time it takes for this to happen is about 20-30 ms to the upper extremity and 35-50 ms to the lower extremity. However, if fighters are throwing punches and kicks at faster-than-light (FTL) speeds, 20 ms is too long of a time to dodge.
In reality, our brain sends a signal to our spinal cord which sends a signal through a nerve to a muscle to move a specific part of our body. These times have been measured and it has been reproducable in several studies that are published today in peer-reviewed journals. We can stimulate regions of the brain using transcranial magnetic stimulation and move specific regions of the body like a thumb or toe. (I do this in my lab in real life.) The time from stimulation to movement is the latency. However, this is not the case for all movements. Sometimes, the spinal cord can send a signal to a muscle directly without the brain. Even the fastest reaction time 5-10 ms, is still too slow! FTL speeds are above 300,000 m/s while the fastest conductance velocity of a nerve measured is about 70-75 m/s. Our nerves are limited by its conductance which is based on the myelin sheath that surrounds it. Think of a wire with electricity flowing inside it. The thickness, length, and the material the wire is made out of all contribute to how fast electrons can move inside of it. For nerves, it's the same thing, so we cannot change the nerves conductance properties. We are born with it! In case you need proof that characters in DBS are FTL, it was told to us in the DBS manga that Jiren can move faster than a spaceship to travel to a planet. The speed of an average spaceship in DB has already been calculated before: viewtopic.php?t=25990 Note that all previous assertions that claimed characters were FTL had no proof. It is not until Jiren's statement we had direct infallible proof.
Therefore, for the sake of the DB verse which is a fantasy, we have to assume that the delay is actually 0 ms! That is, the body moves by itself instantly with no input from the brain at all! This is what is colloquially known as "muscle memory". In real life, muscles can't store memory. It's our brain that stores it but through repetitive action and practice, we react faster which gives the illusion that the body moves on its own. If we agree to this, this allows any person regardless of their opponent's strength and speed, to dodge any attack because 0 ms will always be faster as it is instantaneous. Therefore, a weaker person like Roshi who in Chapter 39 of the manga was shown to dodge Jiren's punches, was still able to do so. Obviously, Roshi wasn't using UI, as Whis said himself, "It's a far cry from UI", but its principles are similar. The whole point of Roshi vs Jiren was to demonstrate the utility of UI and show that it's not a power or speed boost. It's a reaction time boost. To give you an example, if Jiren and UI Goku were to race each other in a 100 m dash, Jiren should still win because he's still faster in raw speed. On the contrary, if Jiren and Goku have to press a button when a light turns from red to green, Goku would press it faster because there's no delay for him to react. That is the difference between speed and reaction time. Please do not get them mixed up!
When Whis talks about “It’s because you’re both thinking before you act”. In real life, our brain is always active whether you like it or not; you can't simply remove it from the circuit; it’s part of alive. Even in a coma, there is small electrical activity of the brain. The definition of "thinking" varies but in cognitive psychology, Sigmund Freud defined thinking occurring in three different levels - conscious, subconscious, and unconscious. If we use Sigmund Freud's definition, then it's quite impossible to stop thinking unless the brain is dead. The truth is that we waste too much time deciding what to do consciously that it is faster to react unconsciously. However, unconscious is really where you are knocked out like in coma and the body doesn't move. It is much more likely that ultra instinct occurs at the subconscious level. The reason I say this is because Goku, Beerus, and Whis are still shown to be partially conscious and even talking while in UI. Subconscious allows you to still be partially awake. In fact, in Chapter 39 of the manga, Whis confirms that it is the subconscioius state so this theory is confirmed! Sleepwalking and dreaming all occur subconsciously. Your dreams are just a manifestation of random memories in the subconscious. Thinking causes a delay and this limits speed. In Naruto, when Rock Lee gets drunk, his reaction speed and movements get faster, why? The alcohol is causing sedation which is limiting is ability to think consciously hence his body is just reacting on its own. It's almost the same thing. This is the basis of what ultra instinct is about why Whis wants Goku and Vegeta to achieve it.
So if we still need the brain, can we reduce reaction time and still teach our body and muscles to react on their own? You can, by practicing a specific response to a specific stimulus over and over. This is what is done in martial arts where a single move is practiced thousands of times, even if that move is simple – until that move is natural. This state of mind in martial arts is known as Mushin. The reason this works is that the brain is forming synapses between neurons and hard-wiring this specific movement. Normally, the first time you learn something, the brain sends a signal between neurons, but the strength of the connection between neurons is weak. The more you do it, the stronger that connection becomes until it is natural. This is how memory is encoded in our brain. The notion of “muscle memory” is false. The muscles themselves do not contain any memory, your brain does. However, the term developed because through practice, reaction time was faster which gave the illusion that one is not thinking and just reacting. You’re still thinking as the brain still needs to send a signal to your muscles to move. The only difference is that it’s subconscious and your body will react on its own without you consciously having to give it instructions to do so. The brain is still involved.
Ok, so that explains the "defensive" portion of UI where a fighter can continually dodge any attack. What about the "offensive" portion? This is where we need to discuss the "heat". A blue energy surrounded Goku and Whis calls this "heat". Heat in science is just kinetic energy. Hence, in this case, it would be the kinetic energy that is generated from moving instinctively while in UI. When Goku closes his eyes, that is when he relies less on thoughts and thus, he moves more instinctively, creating more heat. Eventually, he has enough to form the shape of a galaxy. He then absorbs this energy to get Mastered UI (MUI). This DID result in an increase in power for Goku, not just faster reaction time. This allowed Goku to also boost his offense. Additionally, with the aid of dodging any attack, Goku can now land any hit at any location, leading to critical strikes. He did this in the anime against Kefla, by Kamehameha surfing, dodging all of Kefla's attack and landing a critical hit to Kefla's face.
Whis has already shown to use ultra instinct when he was sparring with Goku and Vegeta, instinctively dodging and blocking their attacks without even looking at them. Beerus in the manga was shown to also do this though he hasn’t fully mastered it. Neither Whis nor Beerus had any indication of a transformation or physical changes. For Goku, there are distinct physical changes seen like silver eyes and the aura called “heat”. Initially, we had thought that these physical changes were a result of absorbing the Spirit Bomb but then they were seen again. These physical changes may be exclusive to saiyans or it is the author's way of depicting to the audience that he's using ultra instinct. The "heat" can be explained by the fact that ki is energy and kinetic energy is heat. Then, as Goku achieved MUI, Beerus stated, "That form, that glow". This is when all the Hakaishin stood up. It can be assumed then, that MUI does indeed have physical changes with with a unique aura that can be manifested even if a non-Saiyan achieves it. We have yet to see Beerus or Whis use MUI. All we know is that Angels do possess MUI.
What if questions:
1. What if two people with MUI fight each other? What would be the result?
2. What if Hit used Time Lag against an opponent with MUI? Can the person with MUI still dodge?
Source for nervous system speeds: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5209850/
Source for the definition of conscious and unconscious thinking: https://www.psychologytoday.com/blog/co ... is-thought
https://www.psychologytoday.com/blog/th ... n-thinking