Yeah I looked back through MAC and couldn't find rules. I swear I read some rules, so either in one of the companions or maybe a different game system.
To elaborate, I have studied both aikido and jujitsu. There is always a method of breaking out of a hold, but it can be very difficult depending on how good of a hold your opponent has on you.
Take for example you are face down, your opponent has your hand locking the wrist and elbow. You are in a really, really bad position. But if you understand how to get out, you still have a chance. If you can roll your arm/wrist into the direction of the lock, both your elbow and wrist are no longer locked. Then you can roll out of it.
But given that you are face down, and your opponent has a grip on your wrist and on your elbow, its going to be very hard - so large negative modifier.
The rolling out, and requires you to pull your knees up so you are no longer flat on the ground, or rolling on your side so you can bend allowing you to use the strength in your legs/back (reducing your negative modifier).
I think in Rolemaster someone makes a grapple/locking hold attack. Either they get a hold or miss. If you're successful in getting a hold, how well you succeed on the hold should determine the the negative modifier against your opponent attempting to break the hold. Barely making a hit on someone sounds like a hand hold on someones arm. Getting a crit should provide a greater modifier.
The negative/positive modifiers on the locking holds crit table seem too small to me. Maybe against two opponents that have skill in grappling they might be reasonable, but overall I don't think they work.
The following round after a locking hold I would say the person achieving the hold could try to maintain it and strengthen his modifier, while the one locked would try to break out. So an opposed roll.
I think grappling and locking holds combat is basically make an attack as normal with damage and crit each round. But apply the negative modifier to the subsequent round attacks until your opponent breaks out, or you miss. The attacker continues to attack as long as he wants to. The opponent has to break out or attack himself to break the hold. This gets a little fuzzy on mechanics...
Mal