In the Assess and Correct DVD it`s demonstrated testing hip and knee flexion with the hands under the lower back. In the leg raise test in FMS I think, if I remember correctly, that this is not done. Is this because of trying to minimize posterior tilt, or what?

I can’t speak for the FMS, but your thought process is correct.  In the Assess and Correct DVD’s, we’re using an approximation of neutral spinal alignment during testing and exercise performance.  Placing your hand behind the lower back allows you to monitor your lumbar lordosis to prevent substituting pelvic tilt/lumbar motion for hip motion.  Without monitoring, you may get a false positive or false negative for your hip testing.

The general rule is that you should be able to slide your hand under the lower back to your knuckles (metacarpophalangeal joint).  If you can’t get your fingers under the lower back, you’re flattening the lumbar spine too much.  If your hand slides under too far, your lumbar lordosis is excessive.

Bill