How does a turbocharger work in a diesel engine?

I know enough about mechanics and how engines work. I know how turbos work in gasoline engines, the fuel/air charge is in the cylinder and the turbo forces in more air and thus more fuel. But if a diesel engine introduces only air and compresses it until the last second when diesel fuel is injected at the top of piston travel, how does a turbo work on a diesel if the fuel isn't present until detonation?
VW Turbocharger