So I'm one day away from owning a phottix flash trigger. Found a retailer letting it go for a cheap price...which is good enough for me. Anyway I saw this video http://www.viddler.com/explore/ziser/videos/56/20/ where it explained how the sync speed effects the flash and how a faster shutter speed results in the light fall towards the lower end of the picture..... But the same does not happen when the flash is mounted on the camera...i.e the light fall is equal at all corners as I increase the shutter speed....why is that?? Can I possibly have the same effect with the external flash??

Thanks