Single Leg Kick on the mat is a fantastic exercise for getting you connected to your seat and the rest of your backside. But, it's also one of the most misunderstood exercises in the mat series I think. When I would teach mat classes I would often hear "Why do we do this exercise?" I knew if I was hearing that then I really hadn't cued everyone into no cheating their way through this one. If you find yourself doing this exercise and it just seems easy. Then you're most likely hanging in your shoulder, dropping your stomach onto the mat and letting your thighbones fall in space as you kick your heel to your seat. When done correctly this exercise is anything but easy!
Kick your heel to your seat two times without dropping your thigh.
Prop yourself on your forearms with your arms parallel to each other fists pressing into your mat. Pull your chest forward and your shoulders back. It's easy to hang in your lower back. Lift your stomach off the mat and reach your tailbone towards your heels to help your lower abdominals connect and lift off the mat. Without letting go of anything kick your right heel to your seat two times. Then press your leg down and switch. Repeat 3-5x each side.
As you do this exercise it's easy to let the hips fall side to side and avoid the stretch of the hips. But, if you can work the control and maybe even reduce your range of motion you will get more stretch and strength from each kick. Another thing to notice is where your knee is in space. For me, my knees are not touching the mat when my legs are straight. So, when I kick my heel to my seat my knee doesn't get to drop down and touch the mat. It has to stay where it is. This requires the muscles of my thigh to activate and hold the thigh bone in place as I kick and straighten. You'll know when you are doing it "right." You're seat and hamstrings will speak to you!