What determines how you can move? Your head and your hips. The direction that your whole body can turn is limited by how much control you have of these two points. Your head mobility controls how much you can move your upper body while your hips determine how much you can move your lower body. Don't believe me? Give it a try. Hold your head to one side and try to turn your upper body. Sucks doesn't it?
This bio-mechanic can be turned to your advantage as much as it can be used against you to your detriment. As a top player, controlling your opponents’ hips and head guarantees you staying in a dominant position. That’s why when on the bottom, changing this ASAP is a top priority.