I like to have my girls learn how to hold upside scoop while hanging under the bar--this allows them to gain the muscles needed to keep their butt from dropping. I also like having them do 2 back hip circles in a row to get them used to the continuous motion. Sometimes I'll set up a mat stack or move the bars in closer and have them aim for kicking the high bar or the mat stack with their feet.
I know what you mean about underswings . I spent all year last year with my level 4's working them. I use a jump rope and tie it across from each side of the bar and when they dismount they have to go over the rope with out touching it. I also spot a lot of them. I also did levers for conditioning thats when they start with their chin at the bar without touching the floor and hold then lean back to thighs on the bar hold then back to chin, we did 5 in a row 3 times. one more thing i used to tie a baloon from the high bar and have them dismount and try to kick the baloon. i hope i was of some help!!!
you could also tell them its like a back extention roll the cum under the bar then they shoot their legs out its worth a try tell them to try to keep their hips to the bar then hollow and shoot out to a landing it helped with the level 4s i trained last year