The OHCI PC Plug-in card includes a power cable that ships with a E8491B. What does the power cable do?

The power cable supplies the host PC power which is required by the OHCI card. The OHCI card, in turn, supplies power to FireWire cables that are connected between the OHCI card and the E8491B C-Size modules. The cable power enables daisy-chained, multi-frame systems to operate even while one or more mainframes is powered OFF.