Back drops on a trampoline with a bounce out of it. Then have them work on back drop half twist to their hands and knees, teaching them how to reach their arms to help generate the twist. Once they have a half, work on doing a full twist that way.
Into a pit have them lean back off a block and fall straight back into the pit, staying tight and spotting the ceiling. Then have them fall and once they see the ceiling, they pull their arms for the twist. once they have the timing down (not twisting on the mat, which they tend to do a lot) and can complete a full twist have them do standing backs off a block into the pit, still spotting something. Slowly start adding the twist in after they see their "spot."
I like to teach arms up for a set and then reaching back with the arm that is the direction they want to twist. After they reach back with their twisting side arm, I have them pull the other arm in to accelerate the twist. We exaggerate that motion into a pit out of a round off backhandspring.
I've also put a mini-tramp in front of a pit, faccing away from the pit, with a stack of mats in front of it. So it goes mats, mini tramp, pit. The gymnast stands on the mini tramp with their hands on the mats. you want the mats about chest height. With the gymnasts' backs to the pit, have them bounce to hand stand a few times. This simulates the snap down of the back handspring. We usually go on the third one- the gymnast snaps down and performs a flip into the pit off the tramp. Once they have that down we start training twists.