There are two ways to get into Open Leg Rocker. Both are great and set you up to take your Roll-up on an adventure. I often think that Open Leg Rocker is more advanced than Teaser. Like Teaser you are balancing at the top but unlike Teaser you are rolling back and up without falling or collapsing. Sometimes when I first teach this exercise to students I do not have them grab their ankles but instead reach for their toes and continue to reach for their toes as they roll back and up. Then as they get how the exercise works and how their center helps control their bodies I have them grab their ankles.
As you roll back reach your arms long from your back!
Sitting at the top of your mat with your legs open the width of your mat either bend both knees, grab your ankles and then one at a time or both at the same time straighten your legs until you are balancing in your start position. A more advanced way to get into this exercise is to pull in to your center, roll back like you would for the Roll-up and lift both legs at the same time and grab your ankles.
Once you are up press your legs into your hands and your hands into your legs your hands into your legs to turn on your seat. Your arms are connected to your back and this is integral as your roll back towards your shoulder blades. Your hips will lift and your body will want to let go, collapse and melt into the mat. But, if you maintain straight arms from your back you will not get "stuck" on the mat!
After you feel you have the exercise you can add to it! There are three variations. The first is with open legs rolling back and up three times. Then hold your balance and open and close your legs three times. Finally, close your legs, grab your toes and roll back and up three times.