so what does it do?

it works as a 4th gear in automatics. it's a highway/cruising gear, so that the engine is runnin less rpms, therefore increasing the life of the engine, and also helping to keep gas consumption down and gas milage up.
-John-

You may want to turn it off in a few situations
- Racing. Obviously you dont' want that 4th gear, you want your revs higher.
- Snow/Ice. I turn it off it's real slippery out, you get better traction when your in L or 2 anyway, but maxing out at a 3rd gear is better sometimes.
- Highway passes. Watch how high your revs/speed are/is already, but if your at appropriate levels you can turn it off, blow past someone, and turn it back on.

when your comming off a turn, press the overdrive off as your hitting the gass you'll really feel the extra power.

hmm i might have to try that if i do this will it mess up my tranny in anyway?

Not enough to do any real damage--obviously you don't want to turn if going 100, because if your revs are already to high you'll shoot up into and over redline, which is never good.

It's the same idea as if you were to shift back to 2nd gear going 70; not a good idea.