trapping

[omnifinite]

From what I can tell, a "grab" is a self-sustaining capture (of a limb, for example) with one point of control, while a "trap" is a capture using counter-pressure from two or more points of control.

 

So grabbing someone's wrist would be simply holding it in your hand. Trapping it would be catching it between your bicep and forearm, or something similar.