When one character is pinned by another character's butt.

This is not the same as sitting_on_another because:

• It's possible to pin without sitting. For example, pinning against a wall.
• It's possible to sit without pinning. For example, if a smaller character sits on a larger one, the larger character is not pinned.

See also:

• All pinned_by_* tags
• All pinned_to_* tags