Im not 100% sure on this but I believe on a gasoline engine the propane is used as a secondary fuel for a propane/nitrous wet kit. It still sprays nitrous but instead of extra gas sprayed in as well it sprays in propane. Added benifits of propane is naturally very high octane....like 120 or so. Also it comes out very cold just like nitrous and will not puddle in the intake...so no nitrous backfire in intake. I believe BTR Motorsports (Vipers) has a kit for sale.

On deisel motors I believe it is injected by itsel into the intake and can raise hp immensely due to the nature of the deisel.

On a turbo car like the supra with high boost.....hmmmm...not sure how they are using it....maybe as a secondary fuel injected at WOT to cool the intake charge further and enrich the mixture with a very high octane fuel at the same time. Sounds interesting. Any links to propane products for cars???