Edit: I'm assuming you mean spin hook, and not spin bichagi?
My experience is also that people either get this or they don't, but some people click if you explain one way, some when you explain another way.
Slow motion kicking can really help, as it's impossible to do slowly out of balance.
4 things that I've found can really help communicate the mechanics of the kick around green belt level:
1) Don't let them lean back too far during the kick - people tend to do this to allow them to lift the kicking leg, to compensate for poor flexibility and strength at the hip, which will not yet not be fully developed at green belt.
2) Make sure they bend the knee of the kicking leg during the chamber and after the kick. This allows the kicking leg to be kept tight to the standing leg during the chamber motion, speeds up the spin and promotes balance recovery after the kick contact. The chamber motion should look very similar to back kick.
3) Make sure they are getting a full 180 pivot on the ball of the standing foot, heel pointing to target as the kick contacts.
4) Last but not least - in fact most on topic, push off with the kicking leg as it leaves the ground - the push should be just enough to bring the standing leg perpendicular to the ground throughout the kick. This little push is what puts the weight over the front leg allowing a good balanced pivot, and it also feeds body weight into the kick.
A good exercise to promote these skills is to have a partner hold 2 paddles at head height, roughly one foot apart. From right leg back, the kicker must kick the paddle on the right, missing the one on the left. The fact that they have to 'insert' the kicking foot into the gap forces a reach with the kicking foot, and therefore develops the balance transition.
As a by-product, it also develops a nice tight chamber and a fast, narrow spin hook with little wasted motion. I'd recommend covering each of the 4 points in a slow motion kicking drill with pads.
Never practice spin kicks for more than 15 minutes at a time. That thing happens where if you say the same word enough times it starts to sound ridiculous. You just can't feel if it's right or not anymore. And you get dizzy.
