Powershifting, also known as full-throttle shifting or flat-shifting, is a method of gear shifting used with manual transmissions to reduce the time where the driving wheels are not powered.
Unlike during a normal gear change, in a powershift the driver does not let off the accelerator.
The clutch is briefly depressed while the shift lever is rapidly shifted into a higher gear, keeping the engine in its power band (unlike speed-shifting, where the throttle is let off very quickly, simultaneously depressing the clutch and shifting into the next gear).
In a lot of powershift setups there is also a method of cutting the ignition or fuel delivery in a manner similar to a rev limiter, which prevents the engine from over-revving when the load from the transmission is removed.
Many aftermarket[1] engine management systems provide this functionality as either a standard feature or as an option, usually combined with launch control.