Traditionally, the auto trans, used a "hydraulic computer", AKA, the "valve body", that would measure the engine RPM, vehicle speed, engine load (vacuum) and throttle position, then determine the optimum shift points and hydraulic pressures for economy, smoothness and efficient power transfer (no slippage). Shift kits replace portions or all of the valve body with modified orifices, springs, diaphragms, etc, that shift the balance towards power (hence the higher shift RPM, when the engine is making more power) and more pressure on the clutches and bands to preclude slippage. If your truck is a late model, many of these functions are now done by the electronic computer, still measures and controls the same things, but the changes are made in the EEPROM or "chip". If the change is in the valve body, you probably need the instructions from the kit to identify the springs and orifices that need to be restored to stock, if its electronic, you need to put a stock chip or program back in (some are chips, some are programs, which need security codes to overwrite the existing program) Good luck.
Blacksmith