The Elbow Slip is a "pre-Pilates" exercise that I learned years ago from my friend Carrie Samper. It's an excellent exercise that helps you feel exactly what muscles you need to be using to get through the toughest part of the Pilates Roll Up. It reveals all the spots you are likely to cheat when doing your Roll Up. If you can't keep your legs down here, then you'll struggle when they become more distal. Lose your round shape as you reach your arms forward and you won't be able to find your longest round shape on your Roll Up.
Pull your tail to your heels and pull your heels to your seat.
Setting up as you did for your Half Roll Back, knees bent, legs tight and together, pressing down into the mat. This time walk your feet a little further away from you and prop yourself up on your forearms. Without sinking into your shoulders or puffing your chest open, find a connection of your arms to your back. Reach your tailbone for your heels and energetically pull your heels towards your seat to gain even more activation of the back of your leg. Feel your upper abs pull in and your lower abs pull towards the floor. To begin, without leaning to one side reach one arm forward and tap its own thigh. Pull it back into place. It should easily "slip" back in. Then switch sides. Continue going back and forth. Your body shouldn't change shape, feet should stay on the ground and you should feel your center engage to maintain this shape throughout the movement. If that goes well then challenge yourself! Reach both arms forward and tap both legs and then slide both legs back. Repeat 5-10x. Use this in place of any Mat exercises that are too difficult for you.