It's an old cue that was used to try to explain how to get body rotation on the swing. The problem is you can "squish the bug" by turning your foot without getting any sort of actual rotation. Best to avoid it, and anyone who is still teaching it.
Good way to test to see if a kid does this. Put a ball by the back heel. If the ball kicks out they squish. Fix it, we have them swing with a concrete block or a bucket by the back foot, so they must learn to swing like Mark H posted.
The problem with "squishing the bug" is that you can pivot the back foot this way while not really rotating the hips. I teach proper hip rotation which should result in pivoting the back foot - in other word hips rotate back foot, not the other way around.