This was fun...
ON wet pavement (or dry... your call) get the tires spinning and when you go over the set lockup speed, the transmission will lock up and start spinning the tires real good. As soon as the tires slow down, the transmission will unlock, and slow down to match the speed of the truck.
This happened to me yesterday on a wet road for the first time. Remember, as long as the rear tires are going over the lockup speed, the transmission will lock up regardless if the rest of the truck is sitting still.