The Reverse Sit is my BEST Half Guard pass. It is my A game and is very high percentage.
There is a little bit of perception that needs to be built when you reverse sit. You want your weight on your opponent ( check the video to see that my butt is not on the ground) and you need to bury your chest as deep as you can into that underhook.
One of the things that makes this pass awesome is it is a great way to kill your opponents underhook. This pass is also great because it is difficult to defend against, gives great submission opportunities and is uncommon (although I'm not sure why because it is easy to do and a lot of high level players use it).
Once you have reversed your body it is important to take that outside leg and bring it straight to the butt. You will find that it is difficult to do without the help of your hand. This should not require any flexibility, it is just an odd angle that we dont naturally have muscles for.
Once that leg is up it is important to push your opponents knee to the floor. I have seen this pass messed up so many times by people trying to push the knee away (as opposed to all the way to the floor) and try to kick their leg out. When you do this you have a tendency to lose your base on your opponent and then the next thing you know you're getting swept when that did not need to happen.
Dont be afraid to base out on the side that the leg is being taken, you can use your head and your arm but be sure to stay on top.
Once that knee is on the floor there are a number of options. But mainly I like to underhook the hand (which is almost always on the knee trying to stop you from mounting. Make sure to underhook the arm as close to the elbow as possible so that you can build leverage to separate it from your knee.