Originally Posted by JMC

The injectors are turned off. You are commanding idle with your foot off the accelerator. The PCM sees this and closes off the injectors. The potential energy that is stored up in the truck becasue it is up on top of the hill is transformed to kinetic energy when you are going down the hill. That kinetic energy is what is turning the engine.

Now answer me this; How did you get up the hill in the first place. That is where you used more gas than the level surface travel. Nothing is free.

JMC