Start your sesh with a series of 10 bent leg sit ups, then transition to 15 straight leg raises. (Feelin' the burn in your tailbone instead of your abs? Put some padding under your tush to cushion your booty as you rock back and forth—that's what Kayla does.)
Finish your circuit with 20 ab bikes per side—and then repeat that routine for three full minutes. Your core will feel more chiseled in no time.