How does this work? How can you tell it's happening? What are the benfits?

 

In an auto trans, the engine and the trans have no mechanical link, just a slosh of fluid turning two propellers (turbine/impeller). when cruising at light throttle, the turbine/impeller are not turning at the same speed cause the fact that they arent connected, so the flywheel will always turn slightly faster then the trans input shaft. in order to get a little better gas milege there is clutch plate in the converter similar to a manual trans clutch plate which engages during a light throttle cruise. this mechanically connects the engine/trans bringing the RPM's a few hundred lower, which = better milege.

you can see if its working by cruising on the highway light throttle, WITHOUT letting go of the gas, gently touch the brake pedal with the other foot. if its working properly, it will seem like it pops out of gear....the rpms will jump up a few hundred. another way is to lightly accelerate and count all the gear changes. in a 4 speed tranny (eg. 700-r4) it will feel like its shifting to a fifth gear.

 

so this only occurs after trans in in final gear, correct ?

 

yep...some people hook up a switch so they could apply it manually.

 

TCC lockup can actually happen in any gear except 1st...

 

There's not enough fluid pressure to make lockup occur in 1st, in 2nd and third some conditions have to be met before it engages, speed above x and speed consistent for x amount of time.

Most of the time the only time it locks up on its own is on the highway. In stop and go city traffic, usually it won't lock.

you can force it with a switch, and then have access to added effciency and a bit of power but its really tough on the tranny. You get the benefit of engine braking too, but all said, I'd rather beat on my brakes than my tranny so I run with it on and then when I have to stop/slow down, I go into neutral.