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 is talking about the time it takes for information from the external environment (senses) to be processed through the brain, then think about an action, and then send a signal to the body to perform that action. Note that Whis says 5 senses which is most likely referring to sight, sound, smell, taste, and touch. These are colloquial in neuroscience. There are other senses beyond these 5 like hunger, thirst, etc. but those are internal senses not external. Other external senses like vibration, proprioception, temperature, etc. are part of touch. Then of course, in the world of DB, there's ki as a sixth sense. That's a lot of time wasted if you think about it. Even if we eliminate the time it takes from thought to action (the hesitation or thinking of what move), it's still a lot of time. For example, to dodge a punch, you either see the punch or feel it or hear it, etc.. Then, a decision has to be made how to deal with this punch - block, dodge, parry, counter, etc. 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 25-35 ms to the upper extremity and 40-50 ms to the lower extremity. However, if fighters are throwing punches and kicks at faster-than-light (FTL) speeds, 20-50 ms is way 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 reproduceable 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: http://www.kanzenshuu.com/forum/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 or spinal cord 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 just a power or speed boost, but a way of movement. 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! Though again, for the sake of the DB verse, which has already shown us that UI is also a speed and power boost, not just a reaction time boost, in all cases, Goku would also win that race against Jiren.
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. In fact, when truth serum (sodium pentothal), is used, the whole concept is to sedate the brain so that it's more difficult for higher cognitive functions. That is, it takes intelligence to formulate a lie. Sodium pentothal is a barbituate and slows the conduction velocity of action potentials, so it's harder to think complex problems. So it's easier to just say the truth. In the same way, if a person isn't thinking too much about what their next move will be in a fight, they'll instantly move without hesitation, making their reaction times faster.
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. 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. That is, the full form of UI, does have "That form, that glow", a physical change, so it doesn't mean we have seen Whis or Beerus use their MUIs.
1. What if two people with MUI fight each other? What would be the result?
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