Copied from different forum, nonetheless thoughtful reading.
I agree with most of what that engineer said. You can confirm his findings with a voltmeter or by simply following the tracings on a powered riser.
There IS an electrical connection from the motherboard to the riser, period...
How much power is drawn from the PCI-E slot depends on how the card is configured by the manufacturer, not how you set up your PSU's. A card mining WILL pull between 35-55 W through the riser, depending on how it's configured and how much power is needed. Dual mining also causes more power to be pulled from the riser.
You are partially correct. Yes, the GPU uses power through the RISER, but not through the MB to the riser. I am not disputing that the GPU uses 35w or even 55w through the PCIE slot on the riser, I'm stating that the electrical connection between the MB PCIE slot to the riser is not 35w or even 10w. The 3.3v connection is a low current bus and isn't supplying much power to the riser. The argument here is with the 12V line, not the 3.3v. The 3.3v is isolated from the PSU through multi-phase regulators so this can be shared across PSUs. The MB is not powering the riser, proof of this is if you disconnect the 12V PCIE power to a quality riser, you lose the card connection. The USB cable is a shielded data cable with 24-22awg wires and even if it were a bigger gauge the specs only allow for a 2.4 amp maximum draw at 5V (12 Watt for 5v, 8w max for 3.3v) without attenuation issues to the data lines. If the MB were powering the risers, you would have 420 - 660 Watts running through the MB in a 12 GPU setup. My MB's are drawing less than 90 watts total so I know this is not the case. If your MB PCIE bus is getting pulled for power, you have a problem with inferior risers. Good risers electrically isolate the 12V from the MB, and ties the PCIE 12V power to the 16X slot using only the 12v PCIE connector on the riser. They also use a voltage regulator from the 12V PCIE power to convert to 3.3v. Your pic doesn't show the top side of the riser where this is happening.
Additionally, unlike 12V power which comes direct from the switching PSU, the 3.3v line is isolated from the PSU because it is created from multi-phase voltage regulators on the MB. It is safe to have the 3.3v come from two different sources because the switching part has already been isolated and converted in the circuit. You don't need to worry about voltage potential differences because they would only be in the 0.1v or less range. It would be unlikely that large power draw through the 3.3v would occur unless it was a complete short at the riser from a bad circuit design.