Can someone Teach Me How To Use Blitz?

[Carnage]

The game is makeing me use blitz on vargas but i cant figure out how i press left right left a. do i do it before sabin makes that light thing or during? could someone please explain it to me!?!

[Carnage]

Iv got it im supposed to put it in when the cursors above his head. i just pushed a there then tried to put it in.