It is a cost containment kind of rule to prevent run away spending on something. Usually it is engines, but I have heard of other items falling in to claimer rules likes shocks and transmissions and, like Brad pointed out, entire cars. IMCA is the biggest purveyor of claimer rules, theirs are on engines.
The idea is if someone is spending significantly more than anybody else on an engine to out power the field, then they run the risk of loosing it to a claim. There are usually some pretty stringent rules to execute claims and it is not a free for all buy fest. Typically, only the top three finishers in the main event can be claimed, the person making the claim had to finish the main event somewhere behind those three, you can claim one of those three engines only, you can only claim once a weekend, claims do not include ignition, exhaust, or carburation. Claim values can range from a few hundred to a grand or more depending on the organization and/or track.
If your going to claim an engine, then you better be darn sure that they are out motoring you. Typically in claimer classes, motors are one step above junk and most the money is spent on items you won't loose, which is the premise on cost containment of the program. When I ran these classes we would typically buy junk yard core motors for $150, hone them like mad to reduce taper, slap in the tallest compression height cast pistons we could find ($100 or so-usually swap meet finds), use some JC Whitney seals all rings ($25), new bearings ($50) a summit or Lunati cam kit ($100) and a gasket set ($50). There you go, running engine for under $500, small block chevy, of course, and it might last 4-6 weeks before needing new bearings. We never finished high enough in any main to get claimed, but had people tell us if we did, they would have claimed us. I attribute that to the fact that we could keep up with the next class above us in the straights, but were still dialing in our corner performance. The only trick in our engines was compression. We didn't have a rule against how much we could run, so we always tried to get as much as possible.
We did spend some bucks on a blueprinted distributor and some custom carb work.
