A jitterbug. A jitterbug or grate tamper is sometimes used on slabs or grade, or exterior paving to push down the coarse aggregate particles below the surface.
Jitterbugs cause segregation and bring many fines to the surface. This causes a weak top surface. The only time a jitterbug should be used is for slabs that are placed with a slump less than 1/2″ which is not a common occurence.