Tampers (sometimes called Jitterbugs) push the coarse aggregate in the concrete below the concrete surface and consolidate the concrete.
Most tamping is done with the finisher standing in the wet concrete-though there are roller tampers that can be used from outside the forms.
Tamping should only be done on low slump concrete. With high slump concrete, the coarse aggregates sink naturally and tamping can cause segregation of the aggregates. Ralph Spannenberg, in “Use the Right Finishing Tools at the Right Time” notes that if a tamper is used when the slump of normal weight concrete is greater than 3 inches, a tamper segregates the concrete, creating a thick layer of mortar at the surface. When the concrete dries, this cement-rich surface shrinks more than the underlying lean concrete. Map-cracking of crazing is often the result.