What is the diesel leak off for on an injector?

The diesel leak off is for getting air out of the system should you run out of fuel. It is very difficult to get the diesel engine restarted after running out of fuel as compared to a gasoline engine which is relatively easy. Once you have run your diesel out of fuel, the injectors must be primed for you to have even a chance of getting restarted. So, VW/ Mercedes owners, keep that tank about a quarter.