Kelly Starrett has a real knack for taking important principles of CrossFit like midline stability and putting them into real-world anecdotal scenarios for groups of trainees. This time around at CrossFit Santa Cruz, Starrett is doing just that: explaining midline stability by turning your body into a series of levers.
His main point is that many athletes get into the start position for a rep and then try to stabilize their midline. This may not be possible in every situation. For example, attempting to set your core under a 1RM squat load might prove impossible, and the resulting squat might have less-than-optimal positioning.
Starrett shows how you can brace yourself into a great position before you address a load, which will help you maintain that position throughout the lift.
Additional reading: Your Form Sucks by Dr. John Zimmer, published May 22, 2010.