Often misunderstood and not “taught”, the skill of sculling is used in every stroke and treading water. You really need to try this yourself and think about how to describe it.
Use the ping pong drill to start with, encouraging the fast rotation of the hands.
A basic introduction for head first sculling:
‘Start with your arms by your side, slide up the rib cage until they won’t go any further, then stretch out to the T position. Now slap your thighs”
“Slowly up, sneak them into position then accelerate”
The progression is to keep the arms by the side and only rotate the wrists.
Feet first is harder as it needs the swimmer to use the core muscles to hold the legs up, preferably with the toes in the air.
Keeping the fingers always pointed at the toes, rotate in and out fast little circles.