Rootbeard,
The tab is an elevator vortex neutralizer. It's purpose is to bring the elevator tip vortices to the center of the elevator, and to also centralize and minimize the vortex effect, thereby smoothing the overall airflow off the back of the airplane, reducing friction and overall drag effect.
Actually, that's a bunch of bull. One night in the shop about 1:00 AM I wanted to break up the cosmetic line across the back of the elevator, and I thought it might look cool when the model is finished, so I added it.