PCIe power connectors have a small notch around one of the pins that is missing on the female end of these cables, meaning you'll either have to squeeze the cables together hoping nothing breaks, or physically modify the cable.
Pros
WIDE COMPATIBILITY: Compatible with 6-pin and 8-pin graphics cards from well-known manufacturers ASUS, ASRock, Radeon, Sapphire, EVGA and Gigabyte.