The hammerhead is an interesting maneuver because three forces, slipstream, torque, and gyroscopic precession caused by the aircraft's propeller come into play to a greater extent than in most other aerobatic maneuvers. The effect of these forces is especially pronounced in high-performance competition aircraft.
The beginning of the hammerhead is similar to the beginning of a loop; it can be thought of as performing a quarter loop, which would place the aircraft on the vertical upline. The minimum speed for a hammerhead is the loop entry speed, though the faster the entry speed, the longer the vertical upline, and the more well-defined and distinctive the hammerhead. In a Decathlon, a good entry speed is 160 mph, or about the same airspeed used for the reverse half Cuban eight.
The interesting forces become apparent as the aircraft begins to run out of energy along the upline. The slipstream is usually drawn out behind the aircraft becomes tighter and tighter as the aircraft slows. The slipstream forces the fuselage and tailplane to the right as it tightens, which causes the left wing to drop, so it has to be countered with slight right rudder.