VIFF is an acronym, standing for 'Vector
ing In Forward Flight.' It refers to the practice of diverting the thrust of a V/STOL
aircraft away from the normal 'forward' angle while in horizontal flight, with the aim of performing maneuvers not possible under purely aerodynamic control.
VIFFing (yep, it's used as a verb) was discovered by British and American test pilots working with the Hawker Siddeley P.1127, a testbed for V/STOL technology built in the 1950s. Later, as interest in the concept grew in various militaries, several NATO nations funded the construction of additional P.1127s, which were given the designation XV-6 Kestrel (its namesake, the Kestrel, is a particularly maneuverable raptor).
Pilots found that by 'blipping' the thrust of the vaned Pegasus engine down in concert with maneuvers which involved a pitch-up attitude, they could (in some cases) dramatically increase the aircraft's responsiveness (or decrease its turn radius, if you will). This is of major importance in dogfighting! A V/STOL aircraft can thus turn much more tightly than a conventional aircraft by rolling so that nose-up is into the turn (also known as a bank) and VIFFing to push the aircraft 'up' (into the turn) more quickly.
In addition, it was possible to 'hop' the airplane up along its Z-axis suddenly. Preceded by gradual speed loss, this maneuver could allow an adroit pilot to 'hop over' a pursuing aircraft, to gain a position behind his opponent. This maneuver is enhanced by the fact that on the eventual operational version of the Kestrel (the AV-8A and AV-8B Harrier, in the US variant for example) the thrust of the main engine can be diverted past 90 degrees (up to approx. 98 degrees). This 8 degrees of 'reverse' thrust allows the aircraft to decelerate very quickly, while 'hopping' at the same time.
VIFFing maneuvers are distinct from maneuvers designed to transition the airplane from horizontal back to vertical flight. A VIFFing pilot intends to finish up his or her sequence still in forward, aerodynamic lift flight, having performed some feat of aerobatics impossible for an airplane relying on fixed thrust.