Hockey goalies love getting their legs into awkward positions, and the RVH (reverse vertical horizontal) is no exception. The difficulty with the position is that it puts the hip into internal rotation, a motion that many athletes are lacking. 

This is hip internal rotation:

Test this on yourself, you should have close to 45 degrees. It just so happens that if you spread your pointer and middle finger, it roughly is 45 degrees. With the pointer finger down, see if your shin is able to line up with your middle finger, as shown above. If you can’t get there, you may want to work on improving this range of motion so you can get into the RVH position.

Typically there are two things that limit this motion (assuming you don’t have severe hip arthritis). It’s one part muscle, and another part capsule. All joints have a capsule around them that provide stability and house the synovial fluid inside the joint. These can feel stiff like a new baseball glove and limit range of motion. But before we get into how to address this, it’s important to note that there is something called FAI (femoral acetabular impingement). This is pain in the front of the hip when you squat. This often leads to a tear of the labrum in the hip and can be exacerbated by aggressive hip internal rotation. So if you have these symptoms, consult with a physio before trying these exercises. 

Here is a general stretch for hip internal rotation:

To stretch the capsule, use a thick resistance band to drive the femur into the socket like so:

Also, a muscle called adductor longus (in the groin) can limit hip internal rotation. Start with some soft tissue work using a barbell:

Then do some active stretches of the adductor:

You can use straps like shown, or sliders on the ground. Even a towel on some tile floor will do the trick. 

Finish with some active hip internal rotation:

And always, retest. Go back to the seated position and see if your shin is closer to lining up with the middle finger.