Going into the butterfly for ice hockey goaltenders.
It "looks" easy. But you aren't just kneeling.
Try it. First kneel down on both knees, toes on the floor. Easy.
Then, spread your feet apart, toes on the ground, but keep your knees together. Also easy.
Now with your knees together and feet apart, angle your feet and ankles so that your toes are now pointing forward with the heels and ankles touching the ground.
Many goaltenders need surgery for pelvis problems. The head of the femur gets reshaped and no longer is round. I have this same problem but from doing squats.