yeah the timing is reletivly strict. from what i can tell in order to pass through you have to be in the part of the animation where haku is leaning foward. if you don't start soon enough you'll get caught before you start leaning, and if you start to soon haku will rise from the leaning position right as the sword is over his head. now correct me if i am wrong but i belive that during the time hakumen is leaning forward, the hieght of his hitbox is ruduced, enabling him to avoid projectiles and such that would have hit him in the chest or head.