This is a pretty good shot of the top leg posted back. I think you have the general idea. You shrimp back using your legs and then you get that leg back as a post. In the image you can see that the escape is pretty much over (knee is in) and the top leg is still posted with toes on the ground. This is important and hard to get used to. That leg has to stay back for the entire escape. It is very common to forget about this leg and lift it off the ground. Keep it there until you are either back in guard or manage to get to the back or get on top.
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