There is a special fitting on the tank that is a safety switch.  In the event that gas runs faster than normal (ie if there is a break or breach in any of the propane lines) the flow of propane is stopped.  A special hose runs from the propane tank to the front where there are two solenoids installed.  Both are on switches.  One allows a  small quantity of propane to go into the carb (PCV) as a vacuum sucks it in.  You still use gas but the propane is to ensure a clean burn and lower carbon buildups.  The second solenoid is connected to a switch to turn it on, plus a vacuum setting that senses when the vacuum gets down to a set level.  Mine is set around 7" and if the vacuum drops down and if the switch is on then a higher concentration of propane is brought into the vacuum of the carb (PCV).  This amount is set by the orifice that you specify when you install the system.  The larger the orifice, the more propane you inject.  Thus it helps when you are climbing hills or needing more from the engine.  Otherwise the second solenoid does not come on.  The first one can be left on and it lets in just a small amount. hope that helps. al