EVGA Power Boost
EVGA boards had a problem with the molex conector set up to power more that three GPU's so they came out with this adapter to fix it.
Some board were melting going up in smoke, so this is actually a fix to a failed design.
You get it for free if you own a classified board i think.
The problem was that powerful multi-GPU setups can draw more power than the PCI-E connectors provide. This extra power will be delivered by the motherboard via 24-pin motherboard connector. If too much current was delivered though the 24-pin motherboard connector, it would melt. Some motherboards have a solution for this issue - a supplemental PCI-E power connector which delivers more power to the PCI-E slots. Most motherboards with this feature have a 4-pin molex connection (37.5W) while others have a 6-pin PCI-E connection (75W). The issue is not limited to EVGA motherboards exclusively - any motherboard matched with a powerful mGPU setup can have this issue.
The EVGA Power Boost is a small PCI-E device which has a 4-pin molex connection for extra power to the PCI-E slots. It is sold because unfortunately not all motherboards have a supplemental PCI-E power connector.