Hook your feet under a rack and lay back on a knee height plyo box. The most important technique cues on these are keep the weight as high on your torso as you can and do your best to not go into lumbar hyper extension. On the way down flex your abs so you keep a neutral lower spine. Lay all the way back on the box so your abs re-lengthen then flex back up. Flex your torso before you sit up again.