A friend of mine is going to teach on the subject of high-speed sync flash photography this weekend.
When the subject came up I opened my mouth and inserted my foot (again - I do that a lot it seems). This time it was something like: "you can do the same thing with neutral density filters". Here's my attempt at demonstrating the technique with my least favorite always-available model.
The idea goes something like this: You want to use a large aperture, say f2. You want to do that in the sunshine. The sunny-16 rule would say you need your shutter set 1/8000th. But, you want to use your flash to fill in the foreground and your very fine camera only syncs up to 1/250th.
High-speed-sync OR use a neutral density filter to cut down on the light levels entering the camera. In this case I used a 1.8 ND - that'll hold back six stops.
Shot with a 7D, 30mm Sigma 1.4 at f2, 1/250th, iso100, B+W 1.8 ND filter, with my Alien Bee 800 at 1/2 power.
The original boast on my part was to use speed lights. What I discovered in this experiment is that a single 430EX has to be so close to the subject that it's in the frame - better take Nicole's class after all.
One more thing: Framing and focusing the camera with a 6-stop filter on the front is hard to do. One would need to learn braille...