This is probably unintelligible, but it's my favorite one-session quick fix.
Don't tell them they are working back tucks. Pretend it's a new skill. Don't rush the steps.
Have them lie flat on their backs, arms straight up by their ears. Explain, "this is your straight jump position. Imagine you are doing a straight jump in the air."
Then spot them on a real straight jump (floor or tramp), holding their waist from behind. Tell them to remember the feeling of lying on the floor. Then go back to lying on the floor, and then back to spotted straight jump. Keep repeating that these are the same position. Doing it on the floor reenforces that the arms and head can't be thrown back.
From floor-lying-straight-jump, have them begin to pull knees up and over their head. Keep arms straight and pressed against the floor. Not a full back roll, but close. Focus on knee speed and rotation over the head. This is the set-tuck motion (except lying down)
IMPORTANT: This is a mental thing. Have them watch each other. Have them correct each other. Ask,"was Suzie better or worse this time? What did she do better/worse?" This engages visual/auditory/kinesthetic learners, keeps them mentally rehearsing, even when you aren't spotting them.
Go back to straight jumps adding slight tuck, letting them float slightly back, tilting a few degrees till they hit your shoulder, then return them to their feet. Repeat this until there is no fear of going straight up...and then tilting back. Continue alternating set-tuck motion lying on the floor with a focus on pulling the knees sharply over the top with regular spotted straight jumps, having them tuck more aggressively in their spotted straight jump, which will rotate them to landing on their back on your shoulder.
Go slowly on this, making sure they aren't scared. Eventually they will tuck hard enough to tip onto your shoulder, and then over. If you take this slowly enough, this should be a back tuck with a proper set. Just step out to catch the landing.
Keep spotting firmly on each side of the waist, and landing them on your shoulder for a while so they feel safe. They will do the whole thing very nicely.
If you start spotting on the side like a regular tuck too soon, they will likely get scared and revert back to the head-throw method.