.

 

Simple and short on the OD:
1) OD reduces the RPM's the engine runs at.
2) You can use OD all the time, it will only go into OD above 40MPH.
3) Using Drive just gives some performance & will not go into OD.
4) You can use OD or D around town and it will not hurt the tranny. I use D because the local speed limits are under 50
and OD on the highway over 40 for better fuel milage and less engine wear.

I know someone will go into more detail for you, this the simple explaination.

 

also just another piece is the computer locks up the converter, hence why the lil 4-prong connector that is on the driver side of the tranny is there...

its just a gas saver