Agreeing with Carter that a pelvic tilt will help you with this. If you think about your tailbone scooping towards the ceiling, and your front pubic bone lifting up slightly towards your navel, that will help your lower back elongate. Note: you don't need to feel a lot of shifting. This is subtf to send happy messages to your muscles.
Also, it's okay to ask your teacher for some extra guidance on this.
I'll admit I'm a little opinionated on this, but any yoga teacher worth their salt will be able to talk to you and give you different body cues to help you get into the desired position. IOW, they should have a strong knowledge of body mechanics and have more than one way of explaining poses and movements.
I don't think this has as much to do with your round butt and that it has a lot more to do with the teacher giving good cues to get your muscles and bones in a safe position.
Also, not sure if what you're describing is
a yoga pose, as such, or is something borrowed from Pilates and other core based movement systems.Your reply