AC how 2 do a counter attack

how do you counter someones attack...i did it once and i dont know how to do it again...do i have to swing at a certain time or block at certain time or what..thanks in advance

[spoiler] Keep in mind that you are able to do it during your first stab at combat during the invasion at the beginning, but once you lose your abilitiesyou won't get access to the move again until after you've completed the first assassination. [/spoiler]

Anyway, if you do have it for sure then it's as simple as holding down the right trigger and hitting the X button right as your enemy begins his move. You have to do it right when he starts the move or you'll miss and he'll likely hit you, just a matter of timing, you can practise it at the training area.

you lose the ability to counter attack during your first assassination but you have it at the beginning and the rest of the game after the first assassination. Press X when someone has made an attack move at you. It will go into a quick scene.