It is a new set of springs and spacers (plus gaskets and seals) to change the characteristics of the way the transmission shifts. Those that race, will see less time spent between gears during the shift and improve their times.
The faster shifting means that the clutches will be slipping less and therefore will last longer.
In my case, I also eliminated the Pressure Control Solenoid problem I was having because the solenoid also modulates the shift. But the shift kit bypassed the need for the solenoid.
The Thrasher Kit give you the option of three different settings for how abrupt the shift can be.
GM apparently feels that customers want to have no indication that the car has shifted and make the shifts VERY soft. My car feels much more solid in all aspects even though it just has more positive shifts.
It takes about an hour to install the kit if you've never done it before. It is done by dropping the pan (draining the fluid) and removing the accumulators, replacing the seals and springs (plus a new gasket and filter), then reassembling and refilling the fluid.
2001 Bonneville SE - Dark Bronzemist Metallic - Mods: 180* thermostat, iPod adapter, Digital OnStar Upgrade
Planned Audio System upgrades: Front & Rear 7 Band Equalizer, Amps, Hands Free Bluetooth Cell Phone kit
Planned mods: Front PEM, Ported LIM, Ported Heads, ZZP SR Rockers, Aluminum Gaskets & Reprogram PCM