This might vary with other testers, but the concept is clear - a plug reduces spray pressure. There are horror stories of early technicians using six coke bottles with an injector placed in each. The vehicle was cranked to observe the amount of fuel collected. The atomization of fuel is naturally highly explosive and must be contained for everyone's safety. If clogged, the best way to proceed is to run a can of pressurized cleaner directly in the fuel rail.
Pressure is supplied by shop air.
Because this is a caustic mixture, it is recommended to be done only once. If the injector is not cleansed, it must then be replaced. Many technicians are a bit quick on the replacement idea, since it is a bit easier although more costly to the customer. The cleaning rig is another several hundred dollars. It is probably not of much use in a technology laboratory, unless a significant number of vehicles are diagnosed.