What you’re supposed to do: Think of this pose as a halfway pushup. Basically, it’s where you start in a plank position but then you lower yourself halfway into a pushup position, so you’re really working your core.
The mistake you’re probably making: It requires a ton of strength to stay in this position, so most people end up basically just plopping to the floor—or almost plopping to the floor. The common mistake here is that people let their hips drop because it’s easier. But that puts a huge strain on your lower back, and it also means that you’re not working your core as much, which is one of the main purposes of this move.
The fix: Concentrate on keeping your elbows tucked in by your ribs at a right angle, and keeping your shoulders down—not by your ears. Both of those tricks will help you keep your hips up and your core engaged, so you don’t collapse into a heap on the floor.