Hi,
Just to add to Roger's reply, here is an excellent site for flash techniques that discusses dragging the flash.
http://www.planetneil.com/faq/dragging-the-shutter.html
If you find his site helpful, please donate a few dollars in his PayPal Donate account located at the bottom of the screen. I have not finished reading his site, but have donated because I found his information helpful. I am not affiliated with the website in any way.
Best regards,
Kevin