Instructional Video: Half Roll
I was never one to dominate from the top position. I had a few moves that worked for me in high school and college and I stuck with them, eventually earning a riding point in three-quarters of my matches in my final season.
Knowing that top is my weakest position I am still constantly on the lookout for new moves and ways of riding. This Half Roll is one I picked up post college and I really like it. Give it a try, and you may too.