Time to youtube slow flash sync and see what this is?
On a side note I was shooting the other day mid afternoon with good sunshine and wanted to decrease the ambient light to get more of a pop out of the scene. So I dropped my shutter speed fairly low and used flash to expose for my model/friend. I was concerned that I would get motion blur but the flash seemed to stop motion and the picture of the subject came out very clear with no blur. Is this an effect of using flash? If i was shooting without flash im pretty sure I would have had motion blur.