Shimmy is when the tailwheel becomes unstable and oscillates from side to side, or rotates through 360º usually during a landing roll. This can be an issue on all tailwheel aircraft. It places immense stress on the aircraft structure and can cause directional instability.
Shimmy is usually related to the tailwheel kingpin angle, which needs to be reclined slightly. Reclining it too much will make it overly stable and difficult to maneuver properly.
The issue is exacerbated by the fact that a heavy aircraft with an aft CG tends to incline the kingpin angle forward.
Consequently, if landing in the 3-point attitude and holding full up elevator deflection (as is often taught on other aircraft) the tail wheel is then loaded in a similar fashion and can also lead to shimmy.
Using a Tail-low Wheeler landing tends to lower any chance of shimmy by first keeping the tailwheel clear of the ground, then by reducing the load on the tailwheel when it is on the ground by holding forward (down) elevator.
