A grab is the opening of a wrestle rather than a blow, and the grabber declares which of two things they are attempting before any roll is made:
Take an object held by the target in the zone struck — a left or right arm — pulling a weapon out of the hand that holds it. The hit location within that zone is irrelevant; it is the limb you have hold of that matters. Disarming a swordsman ends a fight about as reliably as wounding him, and rather more cheaply.
Hold that zone immobile for a round, which is how you pin the arm holding the knife while somebody else deals with it.
Winning the Melee test only earns the attempt. Both combatants then make an opposed Strength Trial (d6 + STR), at +3 per Impact Tactical Advantage, −2 if you have only one hand on them and −3 off-handed. The manoeuvre happens only if the grabber wins the Trial as well; otherwise the grab simply fails and nothing at all occurs.
A hold that lands forces the target to Pass on their next turn, though they may still defend if able. On the grabber’s next turn the Strength Trial repeats: win and the hold continues, lose and it is broken. Holding a stronger opponent is therefore a losing game played one round at a time.
Only one Strength Trial is made per opposed Melee test, and only a combatant who initiates one and wins it may inflict the special effect. A counterstriking manoeuvre therefore cancels the attacker’s — whoever won the Melee test gets the Trial.