An automatic transmission's main focus is smooth shifting between gears. To accomplish this it often goes into two gears at once while shifting up, which is known as a shift overlap. In these cars, it is a kit that can reduce or eliminate the shift overlap. It will also reduce wear because the transmission won't be trying to drive in two gears at once.
Typically, adjusting the shift 'pattern' in an automatic gearbox involves interrupting the signal from the ECU to the transmission (usually with a switch, though the adjustment can be made permanent). The resultant shift pattern is usually very aggressive (it will 'bang in' to gear) thus making a car a handful in the wet as well as opening the possibility of damage to already weakened transmission internals. It is advised that you service your transmission more thoroughly than usual if you undertake this adjustment.