I recently decided to purchase a Paul Buff Einstein system. I have a 1D Mark IV and a 1DX that I use daily. I often shoot sports in the evening using a slow shutter speed and rear curtain sync with my 580 strobe. I couldn't find anything in the Paul Buff material about how to fire their products rear sync so I called Paul Buff tech support. They said it was a function of a camera setting. The Mark IV has a menu setting for "rear curtain sync". The 1DX does not have the same menu but it does have a section for "external speedlight control". If you go to this menu, the 1DX does not recognize anything but a Canon speedlight (as far as I can figure). So I called Canon Professional Services. They got out a 1DX and confirmed that there is no menu ability to set a non-Canon light to rear sync. The guy I spoke to was amazed that this feature was missing from their flagship camera but was available on the previous model. He entered a memo to those that address customer concerns with the hope it will be resolved with an upcoming firmware update. He agreed that this should be a feature on this camera. He also said that the best way to get the attention of Canon was to have other 1DX owners call or write to Canon Professional Services and let them know that you want the ability to set your camera to rear curtain sync for use with non-Canon lighting systems.
So my plea is that any of you 1DX owners (or potential owners) call Canon Professional Services and ask that this oversight be corrected. Here is the number for CPS and thanks for your help: 1-888-CPS-4540