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Recent story...
One of these devices was installed on a Polaris ATV (valued at $8k) that was used on a construction site. The construction site manager was notified online that the ATV was moving at about midnight, and it showed-up some distance away an hour...
I was pokin' around and found the following link:
HYDRAULICS
The links on the left-hand-side provide some simple diagrams and animations of various hydraulic components. I found this useful in understanding a few points.