Re: bounding box jump technique
Your arms are very important here too. Learn to time your arm swing with the jumps ... back on the way down then up and in towards your center on the way up - it initiates your rebound. Watch your knees too - out not in (both up and down).
Even do it with your 45# plate. Over swing those arms. Try to land light too.
For a box, I keep my chest and head up, but do try to keep my chest over the edge of the box on teh way down. It seems to help me greatly (I'm 6'2" 230# so it may be a big-guy thing .. I don't know). It feels like I have a very slight fall/lean forward as I jump off the box.