Originally posted by Twarp FWIW, if someone with shaky hands can freeze (normal) frames with a flash sync of say 1/125, panning with a shutterspeed of 1/6000 seems as useful as washing your car and then drive into a mudpool. With the speed of this car you would also have to jerk the camera, which might not be a good combination with SR, if good practice at all when releasing a shutter.
Flashes freeze action because the "flash" of high intensity light is very quick, 1/10000th or quicker, the Shutter speed is often irrellevant to freezing action with a flash.
The slower "shutterspeed" for Flash sync is generally the fastest speed that the complete sensor is completely exposed all at the same time, which is nessecary for flash photography to work correctly. IE when you take a picture regardless of the exposure time (IE what is generally called "Shutterspeed") the actual speed the shutter curtains themselves travel is always the same, the "shutter speed" is the amount of time the sensor is exposed to light. For slower exposure times the leading shutter curtain opens and travels all the way to the bottom exposing the sensor to light, after the correct exposure time IE 1/30 or 1/60th ect ect the trailing shutter curtain starts on its way down, shielding the sensor from light. There is a point (with Pentax around 1/180th second is the time it talks for the shutter curtain to travel the distance from top to bottom, so the moment the leading shutter curtain reaches the bottom the trailing curtain starts on its way down, so 1/180th of a second is the fastest "exposure time" that can be produced where the complete sensor is completely exposed to light all at the same time, thus maximum flash sync speed . To get faster exposure times, IE 1/250th, 1/1000th 1/5000th ect ect the leading shutter curtain starts on its way down, the trailing shutter curtain does not wait for the leading curtain to go all the way down to the bottom, it starts its way down exactly the "Shutter Speed" time after the leading started. So for a 1/1000th sec shutter speed, the leading curtain starts to open and then exactly 1/1000th of a second later the trail curtain starts to close, this actually creates a thin "band" of light that travels down the sensor, thus only giving 1/1000th of a sec light to the sensor as it travels down the sensor. the entire sensor at high shutter speeds is never completely open to light all at exactly the same time.
Sooooo, with flash photography, due to the flash of light being only say 1/50000th of a sec the entire sensor surface must be open for that 1/50000th of a second of light to hit the entire sensor , thus the 1/180th of a second maximum flash sync speed. This is why if you do use a higher shutter speed with a flash than 1/180th of a sec you will get the "letterbox" image where most of you image is very dark and blurred and there is just a small band with the correct exposure present which is when the 1/50000th sec of light was fired. A lot of people think that their equipment is faultly when they get this, but its just because they are not using a faster shutterspeed than the maximum flash sync speed.
Thats why a person with shaky hands can feeze action as they are actually shooting with the speed of the flash of light 1/50000th not the 1/180th of a second flash sync speed.
Still, doesn't explain what went wrong with the OP's picture as no more info has been given to the exact shooting style used when he took the picture, if he was panning or not, but as some say SR could have been working against him as well.