Grappling: Fatigue and Damage?

By revak, in Dark Heresy Rules Questions

Hi there!

I noticed that when you control a grapple, you can chose to inflict normal unarmed dmg (1d5-3+SB) and one level of Fatigue.

My question now is, does the opponent actually have to take wounds in order to gain a level of Fatigue or is the Fatigue automatically gained plus a chance to do dmg?