The Science of Propane
At normal room temperatures, propane is in a gaseous state, since its boiling point is a chilly minus 44 degrees (boiling point simply being the temp at which any given liquid changes "phase" to vapor). But much as automotive cooling systems are pressurized to raise the boiling point of their water/antifreeze mix, so is propane pressurized to keep it in a liquid state at typical ambient temperatures. Release it from that pressure and propane immediately vaporizes, expanding and cooling in the process. (Think of what happens when you open the valve to see what's left in your barbecue's propane tank.)
On older propane conversions for carbureted vehicles, that phase change to a gaseous state had to occur at the mouth of the carburetor, after which the mix of air and propane gas had to travel the length of the intake manifold runners and cylinder-head ports before reaching the combustion chamber. Without getting too buried in the science, these systems could be hampered by cold-start issues and had other inefficiencies.
Today's liquid propane injection systems, as employed by Roush, keep the propane in a liquid state right up until it exits the tip of the (specialized) fuel injector, immediately upstream of the intake valve. Like any form of fuel injection, this improves fuel efficiency but also helps make propane cold-start issues a thing of the past.
More propane tidbits: