Over time, the transmission fluid breaks down and can no longer provide the lubrication needed by the many moving parts of the transmission. In fact, contaminants in the fluid can cause control valves to stick, and bearings and clutches to stop working correctly within the transmission.
A transmission flush is more thorough than a drain and fill. A transmission drain and fill is done by removing the drain plug, draining the old fluid, and then refilling with new fluid. While this is beneficial to the transmission, the fact is that there is still a significant amount of old fluid that doesn’t drain from the transmission. This means that the new fluid is immediately mixed with old fluid, thus reducing it’s effectiveness.
A transmission flush will actually remove most of the old fluid, so it can be replaced with new, fresh fluid. This is done by using equipment designed for the purpose. It can replace the fluid in approximately 10 minutes, usually with the engine running and in “Park”.
Conditioners may be used when the fluid is replaced, thereby extending the life of the fluid, revitalize the o-rings and seals, and make for a smoother shift. A conditioner might not be absolutely necessary, but if can help your transmission it may be worth the extra cost.