Overbearing
To overbear an opponent, a normal attack roll is made. For every level of size difference (1 if a large attacker takes on a medium defender, for example), the attack roll is modified by 4 (+4 if attacker is larger; -4 if defender is larger).
The defender also gains a benefit if it has more than two legs: a -2 penalty to the attacker's roll for every leg beyond two. There is no penalty to the defender if it has no legs. A lone orc attempting to pull down a horse and rider would have at least a -8 penalty applied to the attack roll (-4 for size and -4 for the horse's four legs).
If the attack succeeds, the opponent is pulled down. A character can be pinned if further successful overbearing attacks are rolled each round. For pinning purposes, do not use the prone modifier to combat (from table 51 ph)
If multiple attackers are all attempting to pull down a single target, make only one attack roll with a +1 bonus for each attacker beyond the first. Always use the to-hit number of the weakest attacker to figure the chance of success, since cooperation always depends on the weakest link. Modifiers for size should be figured for the largest attacker of the group.
A giant and three pixies attempting to pull down a man would use the pixies' attack roll, modified by +3 for three extra attackers and +8 for the size difference of the giant (huge) and the man (medium).
