OD button EXACTLY what does it do?

[Wolfracer]

At any constant velocities below 55mph ish, pressing the OD button causes the revs to rise slightly, despite not altering the throttle position.
I was lead to believe that OD was just a 4th gear, and the button was there to stop it "searching" when you are pulling a heavy load up hill and might be flipping between 3 and 4.
The rise in revs is not a kick down, its only 200 or so rpm and is more akin to the idle up function.
Can anyone enlighten me as the search function didn't find anything, but no doubt, once I post I'll see loads of info down there

[TonyN]

It's fourth gear, but the converter unlocking and then locking again when you press it makes it feel weird.

It's good to use when towing up hills, as if it kicks down to 3rd the converter won't lock again till it changes up to 4th again. If you turn o/d off you get lockup in third, so tranny runs cooler.

[BUSHWHACKER]

Overdrive is like a 1/2 gear, (3 1/2) lock-up is fourth

[Wolfracer]

It's fourth gear, but the converter unlocking and then locking again when you press it makes it feel weird.

It's good to use when towing up hills, as if it kicks down to 3rd the converter won't lock again till it changes up to 4th again. If you turn o/d off you get lockup in third, so tranny runs cooler.

Thanks, still a little confused though... why the rise in revs?
So I'd get engine braking in 3rd with OD off?
Also, why does it run cooler? does the engine run at lower revs when locked for same road speed?

[Wolfracer]

Overdrive is like a 1/2 gear, (3 1/2) lock-up is fourth

How does that work then? I'm intrigued and confused.

[TonyN]

Not sure how much the converter has to do with engine braking, the Surf auto is pretty good compared to some, especially offroad at slower speeds.

When the converter is locked the AFT fluid isn't working so hard, is its not doing all the driving while the converter slips like when it's unlocked.