Scroll down to the explanation at Figure 3.
"Figure 3 shows an SATA power adapter with a 15-pin connector as opposed to the customary 4 pin connectors in parallel ATA. The new 15-pin connector may sound as though it would be a hindrance in comparison to the older ones but the two connectors measure almost the same width. The reason for the 15-pin connector is so that different voltages are supplied to the appropriate places. In addition to the customary 5v and 12v wires, new 3.3v wires are included for the new devices. 9 of the pins provided are for the positive, negative and ground contacts for each voltage. The remaining 6 connectors are for the hot-swappable feature of SATA, designating an additional two contacts per voltage for this."