The propane is injected as a vapor before the turbo. With mine, it is a lean air/propane mixture that is injected into the air intake, before the turbo. Boost pressure is picked up by teeing into an existing boost gauge, or by drilling and tapping into the manifold. It makes more power with a turbo diesel due to the increased air.
It is easier and much more driveable to control the flow of propane with boost pressure, as opposed to all or nothing, like other manufactured and homemade systems. The proper tank in the proper orientation is also very important.