My favorite He-man comic: http://www.penny-arcade.com/comic/2005/12/16

haha...i love PA, if you couldn't tell from my fruit f-er av :D

My other car is a Mitsubishi Diamante and it has one of those "smart" 5 speed autos, when I am coming down hill and the car senses that I am pressing the brake pedal too often it will drop from 5th to 4th and if its really steep it will go into 3rd so I dont have to press the brakes so much and to use engine brake insead.

what?

The braking effect is not from negative work done by compression because that negative work is returned in the expansion stroke.
The braking is done by the throttle by creating a partial vacuum.

Since diesels have no intake throttle to regulate load, and exhaust brake (throttle) can be added. or a 'jake brake' can be fitted.