Issued a verbal instruction and then, if you didn’t do it exactly perfect it would be tap-tap-tap-tap on your helmet. Honest.
Training in a tandem seat aircraft on a particular squadron it was said that a foul up would have some instructors undoing their straps, leaning forward out of their seat and belting the student on the head with a knee board. No personal experience, I made my share of foul ups, must have had gracious instructors. Rumours abounded of instructor antics, one that had currency at the time (1967) I never believed, until I came across the following on the Tube a couple of years ago.