Full disclosure, I own 88 shares of Allison (ALSN) which has gained almost 39% since I bought it, about three years ago if I recall. Ok investment I suppose.
Allison has a number of interesting and useful videos on Youtube. If you have a large diesel pusher with an Allison 3000 or 4000 you may find this video useful. It is on the operation of the shifter and covers extracting codes, fluid level, etc. Its 20 minutes long, so beware.
This is a decent overview of the Allison 1000-3000 internals, not too boring and does show the retarder function and operation at 9:10 in the video.
This is how an Allison 3000 internals go roundy-round. Very involved but good animation. They do show the retarder turbine but do not discuss it.
Skip this if you are sleepy or easily bored!!!!