The early ones used a propane tank as the "fuel supply". Two meters showed where the needle was at when you got a good needle. With weather changes, altitude and other factors you could adjust the carb to where it needs to be for existing conditions.
There is a newer electric version from Boris Mazor that is much smaller, almost hand-held in size that uses a tiny air compressor in it. I know of a guy who has one.
The early ones ran around $200.00 as the instruments aren't cheap.