propane is used along with nitrous, when n2o is injected and burned it causes an instant lean situation in your engine because it is an oxidizer, thus causing problems because it makes a much hotter fire, the propane gives you the extra fuel needed to keep your air/fuel mixture from becoming to much air and not enough fuel,(if you have more oxygen you can burn more fuel). burning more fuel gives you more power. if you just run the n2o you have to richin up the fuel/air mixture alot to keep the temp of the fire down so it doesnt cause damage, most of the time the mixture is so rich that your regular perfomance goes down because the engine doesnt get enough air until you hit the nitrous, thus use propane to solve air/fuel problem and get maximum performance out of your ride both when running on happy gas and when not.