What is a Grave jumper/ Pop Up Head?

A pop up head is a head or skull that is mounted on an air cylinder.   If I'm correct, a grave jumper is a head or skull pop up that comes from behind a gravestone.  That may be a simple description, but some grave jumpers are heads , shoulders, and some have arms attached.  In 2011  the wonderful Passport to Dreams & New blog had a post on the history of the pop up heads in the Haunted Mansion.