When shooting Laura and Ryan’s wedding my second shooter pointed to my off-camera strobe and asked “why is the display flashing?” I had no answer and became a little concerned. So, I adjusted the strobe to it’s 90 degree position and the flashing stopped. I then adjusted the angle of the tripod so I could still bounce the off-camera 580EX II and carry on.
For weddings I typically shoot one 580EX II on-camera and one set to full power off-camera when shooting receptions.
After the wedding I did a little research and found this video which explains the issue nicely.
Thank you Phil Steel of http://www.steeletraining.com
Feature image from http://www.the-digital-picture.com