Kick-back is an event which is mostly associated with older bikes fitted with a kick-start and can occur when starting or stopping an engine. It is caused by a piston approaching TDC (top dead centre) without enough energy to overcome compression. The usual result is that the ignition sparks and the mixture ignites sending the piston in the reverse direction which through mechanical connection would send the kick-start with some considerable force back to the beginning of its stroke. This usually happens with your foot still attached to the kick-start, at worst this would lead to a fractured/sprained ankle or at the very least severe bruising.
Electric starts have stopped the personal injury and programmable electronic ignition has gone a long way to prevent this happening, but engines still kick-back under certain circumstances. When this occurs with the Hinckley twins, the starter idler gear takes the full force of a kick-back event and the idler gear shaft can break out of the idler gear boss causing damage to the lower crank case and the outer clutch cover. The normal route to repair this damage would be to replace the lower crankcase and clutch cover through a complete engine strip down. This is a very time consuming task and will on average cost between £2500 - £3500 for a dealer to supply the parts and carry out the repair.
These products are recommended for your bike.