It doesn't weaken the defence, counter-attack means your defence allows the opponents to come at them, commit themselves to their own attack, and then, once the ball has been taken by your side, quickly passed upfield, catching the opposition out of position.

Use Force Counter with defensive formations like 4-5-1V with (in this case) long passing, focus attack down flanks. You'll need good, fast wingers/other forward players to make it effective.