He was probably referring to cleaning (scrubbing, rinsing off, etc) the bottom of the plate prior to putting it on a rack that was about to be pushed through th

He was probably referring to cleaning (scrubbing, rinsing off, etc) the bottom of the plate prior to putting it on a rack that was about to be pushed through the dishwasher. The bottoms would be clean at this point, but it would be wasted time to spend extra seconds clearing visible food from the bottom of a plate.