Very useful for sloping or vertical panels, the catch cup sticks magnetically to the panel under the area you are working on and catches the surface trace if it should fall off, so it doesn't get lost on the floor. It is also very handy to hold the trace for initial pick-up so you don't need to use your fingers to handle the small bb, which can be tricky.

See video link at http://youtu.be/dq27jW1kKrs