Leaf shutters like the one on your Bronica have no effect on what the flash is doing. Shutter speed is only adjusted to change how much ambient light hits the film. Indoors it will have a negligible effect at hand holdable speeds with 100 ISO film at apetures like f/8. What you can do is shoot a test roll without a flash. Shot the camera indoors at f/8 1/500. Now do it at f/8 1/60. Then see what the difference is. They are booth going to be very underexposed. The 1/500 of a second exposure more so. Sometimes you may want ambient light to play a roll. Look up "dragging the shutter." And of course there is fill flash where ambient light is doing all the heavy lifting and the flash is just eliminating some shadows.
Originally Posted by Truzi
I actually use the leaf shutters to my advantage when shooting a subject with a window behind them. You can bounce flash the flash the flash off ceiling to illuminate your subject but set the shutter speed to correcty expose the brightly lit scene behind the subject. So even if the outdoor scene outside the window requires a 1/500 exposure with a leaf shutter you can still get that exposed correctly and use a flash. On some cameras that only sync at 1/125 or 1/250 of a second at most you are screwed. The scene outside the window will be overexposed. Of course you can stop down your aperture but that might mess up your bokeh and depth of field. It will also require more power from your flash to properly illuminate the portrait through a smaller aperture.