The 90/90 rotation is a trunk mobility exercise worth doing well. Small setup details make a big difference in what you actually get out of it.
Setup
Lie on your back. Place a towel between your knees, arms out to the sides at 90 degrees. Lift your feet until both your hips and knees are bent to 90 degrees and hold that position throughout.
How to Do It
Rotate one palm down toward the floor and let your knees fall toward that side, going as far as you can control. Return slowly to center, then flip your palms and rotate toward the other side.
The palm position matters. On the side you’re moving toward, press your palm into the floor for stability. The opposite palm faces up to give that shoulder room to move.
Two Things to Get Right
Keep the opposite shoulder blade on the ground. As your knees go left, your right shoulder blade stays flat. This is what lets your trunk rotate rather than tip as a single unit.
Exhale through the hard part. As you reach end range, a long slow exhale lets you access more motion. Hold your breath and you’ll cut the movement short. When it gets difficult, breathe out slowly and let that carry you deeper into the rotation.
What Goes Wrong
The most common mistake is letting the opposite shoulder blade lift. When that happens (especially if you turn your head along with your knees) the whole trunk moves as a block and the rotational stretch disappears.
The other one: skipping the towel. Without it, the downside knee tends to drop open, which makes the movement look larger than it actually is. Keep the towel in place so you’re working with an honest range.
TL;DR
Towel between the knees. Hips and knees at 90 degrees. Palm down on the side you’re rotating toward, pressing into the floor. Opposite shoulder blade stays down. Long slow exhale at end range.