To cut a countersink on the far side of a workpiece by feeding a tool through a pre-drilled hole. The cutting edges deploy outward once the tool clears the back of the part, allowing it to cut a chamfer as the spindle pulls back. It is commonly used to deburr or prep the back of a hole for flathead screws without flipping the part.
Heard on the job
I'll program a back-countersink pass to break the edges on the inside of the tube.