AVR-USB-JTAG and USB power consumption

The Olimex USB JTAG programmer makes the following claim:

No need for external power supply - takes power from USB port

Could someome explain exactly what’s meant by this? I find that it doesn’t work unless I supply +5V on the Vsupply pin.

much thanks.

just guessing … since it’s optoisolated, maybe they just mean that the computer half doesn’t need its own separate power supply? (which it might if it were an RS232 device.)