Actually, that was in the Cessna 177 Cardinal, and the stabilator had to have the slot added so that it would not stall when at a high "up elevator" deflection. This was a result of them finding out that pilots were doing full-stall landings and winding up with the the stabilator stalling and causing a rapid nose drop while in the landing flare. The slot energized the airflow on the stabilator's lower surface and allowed it to keep working properly throughout the airplane's speed range. Boeing 727 stabilators had vortex generators on their lower surface for the same reason: keeping the airflow attached at high angles of deflection.