She can feel the beat. If you can think of it as hearing with the body? Most likely there was a mic or some other sound device layed on the floor for her so that the vibrations of the tempo were transmitted to her that way.
I have a deaf friend, and if I want to get her attention from across the room, I stamp my foot (perfectly acceptable in deaf culture). It's the same principle. My friend, when she goes out to dance stands very near the source of music. If it's a live band she's been known to ask them to play louder so she can feel the beat better. She too is a very graceful dancer.
Marlee Matlan is a huge role model in the deaf community.