Throw Your Foe to the Ground: This works just like a trip attack except that you don't make an initial touch attack (because you're grasping your foe already) and you and your foe make opposed grapple checks to resolve the trip attack. If you win, you and your foe fall prone in the space you both share, but you're still grappling. At your option you can take a -4 penalty on the opposed check; if you win you break your foe's hold on you and you throw your foe to the ground in a space adjacent to the space you formerly shared. (You stay on your feet.) Your foe's movement does not provoke attacks of opportunity, nor does it count against her movement for the current turn (or her next turn).
If you lose the opposed check, your foe gets a chance to trip you by making an opposed grapple check, just as described above.
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Here are a few optional maneuvers for use against a foe you've pinned.
Throw Your Foe to the Ground: Make an opposed grapple check as a melee attack. If you win, your foe winds up prone in any square adjacent to the square you and your foe formerly shared. The movement provokes attacks of opportunity from enemies who threaten the space your foe leaves, but the movement doesn't count against the foe's speed for the current turn. You stay on your feet in the space you formerly shared with your foe and you and your foe are no longer grappling.
Toss Your Foe: Make an opposed grapple check as a melee attack. If you succeed, you can literally pick up your foe (provided you can lift your foe's weight). Make a Strength check; if your result is at least 10, you toss your foe 5 feet. For every 5 points your Strength check result exceeds 10, you toss your foe another 5 feet, to a maximum of 25 feet.