i just came across this.. this is all i cud get that steering diodes direct the course of power supply.
as you can see in the fig. if V1>V2, green diode wont conduct and load wud work with V1. If V1 goes off, load would work with V2. This idea is used for backup supplies. Consider source1 (V1) to be an AC supply and source2 (V2) to be a backup battery. The diodes steer the supply to power the load.