MA81 is a variation on Parry and only defaults to Sumo/Judo/Wrestling. It is a defense against a Slam, normally, but does have another use.
Even though parries normally imply contact with hand/arm, or in later books a foot/leg (see Leg Parry and Jam) this is different:
"Trip can use many body parts: arms, legs, a hip, etc."
The Hip is an undefined hit location but likely implies the Pelvis, a lower portion of the Abdomen sublocation of the Torso which was defined later in Low-Tech. This would be important to know if using rules for Hurting Yourself or if someone were to use PUA against a Hip-Trip.
What else "etc" may cover is unknown - obviously it could be tricky to allow tripping with literally anything, such as the head or neck. These are short implements difficult to stick in the way of things.
A "free foot" is explicitly the only means of tripping someone who isn't trying to Slam you though, thought doing a Dive which presumably allows other body parts to be used to trip.
The situation of attacking someone not slamming you is as follows:
- someone running past you (not merely stepping) within a yard.
- On a battle map, that’s through your hex or an adjacent hex.
This is very similar to an Interdiction and still counts as a parry. It presumably applies to anyone using Movement Points and moving multiple hexes in a single second, since Step is excluded as a valid target.