It is a very good theory actually. The idea is that with you hips elevated the semen pools at your cervix allowing more sperm to get where they need to go. One thing that a lot of people don't realise is that the semen is just the carrier of the sperm, once inside you, the sperm swim into your CM & are transported that way. So any leakage you have while it will still contain some sperm, it will mostly just be empty fluid IYKWIM.