The Quickshifter, for when you've got to get out of corners that little bit quicker than the other guys. Essentially, a quickshifter insert a switch between the gearchange pedal and the gearchange shaft. When you operate the lever, the switch sends a signal to the control box, which will then cut the power to the engine – either by cutting the power to the fuel injectors, or the ignition coils.
If you only ride on the road, then you don't really need a quickshifter at all. But if you ride on track, then a well-set-up quickshifter will allow you to change gear without shutting off the throttle. This saves time; the airflow through the throttle bodies remains high, rather than stopping and starting again as you close and open the throttle. Power is only cut for the split-second needed to engage the next gear, and while it may not be much slower than a perfect manual change, the sheer number of changes on a track lap mean they all add up to a significant amount of time.
Plus, just one fluffed manual gear shift will cost you loads of time – but the electronics never miss. Since you're using a device that cuts engine power, anything with poor connectors of shoddy wiring risks leaving you stranded by the roadside. Some quickshifters offer plug-and-play wiring looms to simplify installation. Similarly, you need a suitable shift rod adapter – check that one can be supplied for your motorcycle and rearsets.
The most expensive shifters use a strain gauge sensor so the amount of pressure needed for a change can be fine-tunes, some shifters can be incorporated into your engine management ECU, and there are various strategies used to ensure smooth operation; power can be fed back in gradually be switching the cylinders back in one or two at a time.