The most difficult part of the waterfall backbend is the last few inches before your hands hit the floor. That is also the most difficult part of the return to standing. If you can get your legs straight you have a fighting chance of making it back up to vertical!
This drill, we affectionately call “Down-Ups” is an intermediate exercise to help smooth out that transition. You *must* already be comfortable with waterfall backbends to try this! I can be intense on the lower back if you don’t have enough strength to support your spine in a deep hanging backbend.