I'm starting to get the "soft" high fall down-I hit it about 70 % of the time off a kotegaeshi. I have found a great exercise to practice it with. Have one of the class members provide an extended arm. The exercise involves walking or slowly running up and hooking the extended arm with the inside arm and kick the legs up high, then look for the ground to the outside and reach way down. As the hand touches the ground, you lower yourself like a stiff shock absorber by riding the arm down-downside leg should be straight. When done properly this is almost silent.
The biggest difficulty I have with this is the habit (15 years) of wanting to slap on a high fall. When I get somewhere in between the fall is ugly and it does not feel particularly well. When I do the soft high fall properly it is virtually painless. Once I can get my mind out of the slap mode I think it will work nicely. When you go over fast, sometimes it's hard to break old habits. Some of my beginners are picking it up fast since they don't have to break the habit patterns.