Originally posted by Tim Griffiths
I would say that a pin is when you apply pressure to part of uke's body, and the direction he needs to move in to release this is blocked by the mat. A standing kotegeishi finish is a good example of this, and the standard nikkyo/sankyo finishes. Its a literal 'immobilization' of part of the body.
The other case (let's call it a supression) is when uke is not immobilized per se, but cannot get up because they can't get any leverage against the mat, or the direction they can get leverage (their escape route) is blocked by tori's centre.
Good distinction, Tim! I like it.
I guess I usually try for the "control my uke's escape exits" rather than "pinning" them to the mat. It seems a lot more logical to me, at least, and feels to be an extension to the rest of the aikido principles.