Sekiro: Shadows Die Twice | How to Fully Deflect Genichiro's Flurry
Channel: ZonkRT
Date: 2019-05-08

It's a double tap. A very precise double tap.

The short explanation is that Genichiro's flurry isn't tagged as a multi-hit attack, so you're not intended to deflect it like this.

The longer version is that spamming the deflect button causes the window for a successful deflect to shrink with each attempt. This is an intentional mechanic designed to keep the player from just mashing L1 to become invincible. Enemies with flurry attacks such as the Long-arm Centipede Giraffe have their moves flagged in some way to ignore this rule, which allows for the multi-deflect strings necessary to defeat the move.

For whatever reason, the last attacks of Genichiro's flurry do not have this flag. This means that for these four attacks, the window for a successful deflect shrinks more and more. I suspect the last hit is probably a 1 frame (1/60th of a second) input. Maybe that's the minimum, maybe it just worked out that way. Regardless, you're not supposed to be able to deflect the attack like this. It's purely for style. 

This is all conjecture from my experience, I have no idea if it's real. If all you care about is beating the move, just dodge forward and to the right when the four hit sequence begins. You should be able to either deflect or dodge the last hit easily.