It appeared to have deceived Cell, since before Vegeta fires it, he was fully confident in withstanding the blast and calmly stood his ground. His expression quickly changed once it is unleashed, and that's where it becomes more ambiguous as to the circumstances of Cell's reaction.



From what we can see, the attack does not take a single form, but appears to wildly expand from its origin and is then concentrated into a slimmer, more precise point of impact. Is this what Trunks was referring to when he establishes that Vegeta made its range small? And hypothetically speaking, if he hadn't done that, would the blast have exclusively taken its wilder, more expanded form without compressing at the end, or was Trunks' statement just meant to refer to Vegeta controlling and guiding the attack away from the Earth, and the technique would always look that same way?
Taking into account Cell's reaction, he should've been able to sense just how much power Vegeta was accumulating before he fires it, and we know he had much more Ki in his reserve than Vegeta could've amassed in that technique. Judging from the images, could the Final Flash have an effect similar to the Taiyo-ken that blinds the opponent temporarily? It appears to me that it does, I'm not sure how else to interpret those vague specks from Cell's perspective, the attack looks to have obscured his vision some way. If that is the case, he'd be unable to anticipate the attack landing. Concentrating the attack into a precise beam may also give it a piercing effect similar to Piccolo's Makankosappo. I think it is for these reasons that the attack was so effective on Perfect Cell, when power alone shouldn't have been an issue for him. Here's the entry from Daizenshuu 7 for a more official outlook on it, translation courtesy of Herms:
Also, do you think Cell tried to dodge the attack or was the result on Cell's body due to Vegeta's aiming the technique away from the Earth?Daizenshuu 7 'Special Attack Dictionary' wrote: Final Flash
First Appearance: Chapter 384
Category: ki manipulation
People: Vegeta
Special Characteristics: Super Vegeta’s super special attack. Concentrating all the ki in his body into both his hands, he fires it at his opponent. As he concentrates his ki, his hands enter a state of electrical discharge, distorting the surrounding atmosphere and drawing everything in. Its rate of energy depletion is high, and it can only be fired in a state surpassing the limits of Super Saiya-jin. It has enough force to easily destroy the Earth, and Vegeta is able to narrow its attack range by aiming it. This technique blew away the upper right portion of Cell’s body. (Daizenshuu 2, p.214)
