What is a bell bottom pier concrete footing?

Piers are drilled when the ground conditions at the surface are not good for supporting a foundation. An engineer determines from an engineers soil report where the bottom of the pier needs to be. The driller has an attachment that will drill a 36" bell bottom with a shaft size of 12". The pier is drilled, the bottom formed, steel cage dropped in the hole and then filled with concrete. These piers will be placed below the grade beams of the slab.