The trick to getting the puck off the ice / power
With this “shot” you can’t really draw the puck back to build power, and the stick is in an awkward position, however the same basic rules apply in the release.
The trick to getting the puck off the ice is in the SNAP, that means pushing down with the top hand and pulling up with the bottom hand. You wont be able to get much motion here so the wrist snap is important as well. Do it slowly and look back, the toe of the blade should be on the ice, with the heel off the ice. Now roll your wrists over quickly to do a nice little snap, this motion combined with the push of the top hand should get the puck off the ice.
There are a lot of examples of the between the legs goal so I have made a playlist and embedded it here. As I find more examples I will add them to the playlist!