This can be done using a sync cord, infrared triggering device or, if you have a Nikon system, the camera's own pop-up flash will trigger a compatible speedlight--SB800 in this case.
For this shot I used a sync cord and held the flash in my left hand. This is a very common technique and usually when it is employed the flash is, understandably, coming from camera left. (You're holding the camera with your right hand and the obvious default is holding the flash up in your left hand.) As you can tell, however, the light in this photo is coming from camera right. All it took was a minor physical contortion. With the camera to your eye, bring your left shoulder under your chin, extend your left arm out to the right (take a quick peek to see that the flash is pointed at the subject!) and shoot.
A little extra effort and a simple portrait becomes just slightly more interesting.





















This first photo I captured down on the C&O Canal towpath. I noticed a rider coming along and set my camera to a slow shutter speed. As he approached I fixed a focus point on him and followed along all the while making sure that focus point never left his body. I clicked off about three or four shots before he passed. The trees make for a good background and are well blurred to convey actual motion. 