On OBD1 cars, they simply put it on the rollers and put the Gas Analyzer sniffer in the exhaust. If you've got a cat and aren't running extremely rich, you shouldnt have a problem.

On OBD2 cars, they plug the emissions equipment into the OBD@ Diagnostic Port and read for DTC's, emissions readiness, etc. This means that the ECU cannot be switched to an OBD1 ECU like a lot of honda guys do to get Hondata, etc. It also means that you cant just unplug the battery or clear the codes, because 1) they stay in the ECU and 2) because it wont pass a readiness test. It pretty much fucks those of us who tune on OBD2 cars.