I’m still discovering all that I can do with my new speedlight. Earlier this week, I was photographing in the Basic Training barracks on a rainy day; not ideal lighting on a sunny day and definitely low light here. I popped my flash on, bounced it off the ceiling and started shooting. This was boring; so I started dropping my shutter speed. Thus coming up with the really cool shots above.
For some of these shots, I merely panned, but for a couple of them I rotated the zoom lens during the exposure.
The secret to these shots is that the slow shutter speed captures the ambient light and the blurring, while the rear-curtain flash freezes the action.