I thought the question was about the collision_* family of cursor messages (see below).
Bob is right, that triggers send messages when collided with by cursors.
In 0.8.21 there are also these variables that can be used in cursor messages:
When included in a cursor message, these are sent, when the cursor is colliding with a curve, and suppressed otherwise. (However note that there is a bug that prevents collisions with single-line curves from being detected.)