To demo it to the student (on the ground), position the trimmer so that the end is athwartships with the knob on the student's side. Then move the knob 'back', gently push student's forehead back to indicate nose-up movement, move knob 'forward', gently push on back of student's head to indicate nose down movement..... Then repeat - back for up, forward for down.
Mark you, I've also chinagraphed 'U' and 'D' on the headlining to remind the students!