YONGNUO 1M FC-681 TTL Off-Camera Flash Shoe Sync Cord for Canon 550EX 580EX II Upgraded version of Yongnuo FC-681 TTL cord, there are two hot shoes for both two ending side, so you can use this FC-681 TTL cord in two flashes at the same time, and also, it support all functions of Canon EOS Camera, as well E-TTL functions of Canon flash sytem. Using an off-camera flash provides the photographer with the flexibility to direct light from a variety of angles, allowing simple but effective fill lighting for balancing outdoor shots or hiding shadows under harsh indoor lights. An off-camera flash is also an effective primary light source when less direct lighting is desired. If you think the light can't reach your requirement, you can mount other flash into the hot shoe of the one mounted into camera to achieve special lighting for the subject.