All Pilates exercises initiate from the muscles of the abdominals, lower back, hips, and buttocks. The band of muscles that circles the body just under your belt line is termed the “powerhouse.” If you think about how you sit and stand, you will probably find that you sink most of your weight into these areas. This not only causes undue stress on the muscles of the lower back, resulting in soreness and promoting poor posture, but also helps create the “gut” and “love handles” that we all strive so hard to combat.
When performing the mat exercises, remember that you should be constantly working from the powerhouse and lifting up and out of this region. Imagine stretching your upper body away from your hips as if you were being cinched in a corset. This action of pulling up and in simultaneously will automatically engage your powerhouse muscles and help protect your lower back.
This is quoted from an excellent resource called The Pilates Body by Brooke Siler