Ever covered the hose (inlet or outlet) on a vacuum cleaner? It speeds up, less load....the impeller blades are stalled.
The fan speed increases because it is not allowed to move air. So a small amount of air recirculates between the blades and around the edge of the fan. The fan increased the pressure on the outlet side and decreases the pressure on the inlet side, the fan
blades are now operating in a partial vacuum, even if blocked on the outlet side, so there is less load. It has nothing to do with a stalled condition.