I haven't done this yet, but on the 1D4 you can set Av mode and minimum shutter speed, to let's say 1/500s. You then turn ISO safety shift to ON, set your aperture, and put in Auto ISO. You can also set EC, so that whatever value is chosen by the camera for ISO, corresponds to your EC value. This is really usefull for indoor sports and I haven't tried it on the 1DX but I imagine it can do this as well.