I finally discovered how Rachel moves herself around her crib so fast. She turns onto her tummy and pulls her knees underneath her, then kicks at the matress doing a shoulder roll. This explains why she ends up all scrunched at the end of her crib with her legs hanging out. For a baby that doesn't crawl, she sure is strong.
As of yesterday Sam is able to sit back up from crawling around.
It's amazing how fast they change!!