Move this axis down approximately 3 inches (until it is sitting on the shocks).
What does "sitting on the shocks" mean? Does this mean "move this axis until it hits something (maybe bottom?)"?
This sentence is from a manual on how to set up a machine. I couldn't find online version of this.
This is either a reference to a suspension system, or a metaphor for one.
In, for example, a modern car, the load is borne on a system of springs and linkages, rather than directly on the axles. Shock absorbers, informally abbreviated as shocks, are part of this system. The term is something of a misnomer, as they actually serve to dampen bouncing, but that is not really relevant here.
You can raise or lower how high the car sits from the ground by adjusting the suspension springs. Typically, performance enthusiasts want to lower the car to be more aerodynamic (or for more aggressive aesthetics). In an extreme case, the springs would be so short that the car would not rest on them at all, but on the shock absorbers, and the car would be sitting as low as physically possible, hence the metaphor.
Automotive expressions and metaphors are extremely common in English (at least, in North American English). This one may be more technical than most, but it is in the same family as stepping on it or hitting on all cylinders.
