I'm looking at the rules-as-written for grappling in swords & wizardry and I'm pretty sure they are shit. So GM fiat here: I'm going to crib a modified version of the grappling rules from a game I think does a far better job of it (Magic World/Stormbringer)
New Grappling Rules:
-Attacker and Defender each roll to-hit. If the attacker rolls higher than the defender he succeeds in grabbing his opponent. If the Defender rolls equal to or higher they avoid the grappling attempt.
-If the grapple succeeds, The attacker has two choices, pin or trip.
- A trip automatically succeeds and the defender is knocked prone.
- If the attacker tries to pin then the base chance of success is 50%. The two characters compare Strength scores and for every point the attacker's Strength is above or below the target's, he adds or subtracts 5% from his roll to immobilize him/her. (The assumption is that two characters with equal strength have an equal chance of success or failure). Once a target is pinned/immobilized, it can be maintained each round thereafter for as long as they continue to pass their pinning roll.
- A pin can be broken a number of ways. A defender counters with their own grappling check on their turn, or the attacker suffers damage (saving throw allowed to maintain their pin).
Notes:
Multiple attackers vs. a single target: If more than one attacker overcomes a defender's "to-hit" roll, then the Strength scores are summed up and compared to the defender's. The strongest character on the attacking side then rolls the chance to immobilize.
Example: Borric attacks Marcán and rolls a 21. Marcán needs to roll a 21 or better to avoid the grapple. Borric's Strength is 17 and Marcán's is 16. If Marcán doesn't avoid the grapple attempt then Borric
either automatically trips him, or he has a 55% chance to pin him.