Here is a YouTube video of Bruce Gilden
YouTube - WNYC Street Shots: Bruce Gilden
He uses flash a lot, using it on a remote flash cord, using a Leica, and aiming the flash right at people with his left hand while he holds the camera and focuses and ostensibly "composes" with his right hand. He's using a short focal length lens most of the time, probably a 21mm on 35mm film. M-series Leica's have pathetically slow X-synch speed, 1/50 second (yes one-fiftieth of a second)...which was pathetic in 1953 when the M3 came out and was laughable when the M6 cm out some three decades later....his photos have the garish,horrible look of a camera that was never designed to shoot with flash in any sort of daylight,and which might be one of the worst daylight + flash cameras one could buy today, but I digress.
Using the flash off-camera on a "smart cord" or TTL remote cord like SC-28 for Nikon and aiming the flash is a method I have used but mainly indoors,and never on the street.
Watching the Bruce Gilden video for the perhaps the 5th time today in response to your post, I kind of agreed with the first comment posted on You Tube today from a poster named raggedart, who wrote: "what a tool, walking around in a vest and a safari hat in NYC, shooting the same angle and pretty much blindly then calling it art?"