First you need to acquire the shield you will get that pretty early on when you fight your first juggernaut I think then you can start working it on from there what I did was any time it told me to dodge (A) I held (RB) instead you need to hold it not just press it then the enemy so kind of fall backwards also the other time to do this is when it actually tells you to hold (RB). It does not work against rockets or tanks however only melee attacks from what I have found. It can be done against brawlers, so you can easily do it in the red zone after you finish the game.