I saw this the other day on Instagram from Mike Dunn and it was genius--he used a chair to demonstrate but here's how you could do it in the session in the gym--
When teaching athletes to get into the ideal body positioning to prepare for their shot, a lot of the time, their knees will stick out when they lower themselves down to load, which means their weight will distribute awkwardly into the low parts of the quads -- not great for balance.
I suggest on court, putting the back of your hand up about an inch away from their kneecaps and have them "drop" into their hips--their knee should not move forward to hit the back of your hand!
Make it fun and suddenly scream and karate chop them if they hit your hand.
Everyone has fun and now they have a great external cue to and think about. :)