Question:
This is almost 2 embarrasing to ask, but can someone explain how the Jake brake works, I know it uses compression to assist in slowing the vehicle down , but exactly what does it do to the valves to cause this? Someone at work asked me and after thinking about it, it did not make sense. Another co-worker said that it releases compression by opening up the exhaust valves on the compression stroke. but that would allow the engine spin freely and actually increase speed.
Ike
How a jake break works.

Answer:
Thanks Paladin. It was embarassing because im pretty mechanically inclined (believe it or not) and actually went to school for Diesel engines about 13 years ago.
Thanks again
Ike -->

Answer:
Thanks, Paladin: I'd always heard of a Jake break; didn't understand it --- yet, I don't know the purpose of it ... would you or another driver kindly explain it, please?

Answer:
Its purpose is to slow the vehicle. It’s obvious that once a driver lifts his foot from the foot feed (i.e., gas pedal), the engine will begin to lose rpm and the vehicle will begin to slow down. If the driver can increase the drag on the engine, the engine will lose rpm more rapidly, and that’s how the “jake brake” works; by temporarily converting the “engine” into an “air compressor”.
“Jakes” are particularly useful when descending hills/mountains as a means of saving the vehicles mechanical brakes though they can just as easily be used to slow a vehicle on level ground.

Answer:
embarassed
read some of the replies to a post about the same topic,some veteran drivers made some embarassing replies
http://trucknet.infopop.net/2/OpenTopic?a=tpc&s=475098473&f=192098134&m=4263061676&r=6733096676#6733096676
zigzag

Answer:
There's more than just the jake brake when it comes to engine compression for stopping the rig; there's about 4-5 other methods I've heard about. NO, I DON'T REMEMBER WHAT THEY ARE, I'm a darned newbie, for pete's sake. I can't remember all this stuff, you know!
Anyway, for what it's worth (about zero cents, right!)
For the road,
Starkman

Answer:
Flipping the fan switch on gives you braking horse power also. It's not much, I think 5% or so. On long down grades with 80,000lb loads, it helps. We had them in our tanker trucks in addition to the jake.
The fan switch is also used before you start your climb to help keep the temp down longer but it also reduces your horse power but the benefits outweigh the slight power loss.

Answer:
what it does inside the engine is a solenoid pushes down on the exhaust valve during the compression stroke. The 3 different stages is how many cylinders you cut out.