Here is an awesome drill to promote CONTROLLED dissociation. There's a difference between just rotating your body fast versus controlled dissociation activating our muscles. Excessive rotation of the spine, especially with trunk flexion, can cause shearing forces at the annulus fibrosus on the same side as the direction of the rotation. The inner and outter layers of the annulus fibrosus are basically the casing that surrounds the nucleus pulposus (the inside components of a intervertebral disc). When there is a tear in the annulus fibrosus, the nucleus propulus can protrude OUT of the intervertebral disk. This is typically referred to as a disc herniation.
This post is NOT intended to direct you away from rotating and promoting dissociation of the pelvis and the upper body. In fact, we want to provide you with an exercise that will promote ACTIVE CONTROL this movement, rather than stressing passive structures. This is important since we do this movement on a daily basis performing activities, and in lots of sports requiring rotational movements.
Turn your sound on for extra directions and details, but this exercise really promotes activation of pelvic stabilizers, and the trunk rotators. Putting something between the knees to squeeze provides an excellent external cue to isometrically stabilize the pelvis. Just like golf and other rotatory sport movements, this exercise is a concentric, followed by eccentric movement in the transverse plane. If you're doing this correctly, you'll feel your obliques doing a lot of the rotatory work.