Sit on Him

introduced in MA117 requiring a pin and resolved via a Quick Contest. It initially did not appear to require a leg grapple.
TG 25 identified it a a special case of Pass Limb. It became a 2-legged grapple (after all, the buttocks/glutes are part of the leg) on one attack, with a following attack being used to free the leg (presumably shifting the weight from the glute muscle of the leg to sit sit-bone of the pelvis)
The benefit of the new approach is it no longer required pinning as a prerequisite (pinning no longer exists, it is approximated as DX 0 from Control Points). Which makes sense: it should be possible to sit on someone who is not grappled, but they could try to parry or dodge you.
The unique aspect here is that this is an unpenalized attack roll, meaning it appears to ignore the usual -2 base for pass limb, the usual posture penalties AND the -4 to skill for sitting, which would normally total -10 if not for this exception.
Since Pass Limb is defined
:trap parts of your foe’s body with your torso or head, or against a surface.
Sitting on someone is presumably using the torso (the pelvis is a sublocation of the torso) 
there is no posture requirement for the victims, but like with pass limb, they get a bonus to break free if they aren't lying down.
== References ==
* Martial Arts, p.117
** More Actions After A Grapple
*** Shifting Grapples and Freeing Hands
Category:Actions After a Grapple