Really? The Mamiya 645 isn't a leaf shutter? Well that's news to me, hmm interesting. Good luck
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If the synch doesn't match, and the leaf shutter is only part way open when the flash goes off, the film is evenly exposed, but the shutter will perform like a second aperture and potentially reduce the brightness of the light that hits the film.
Sort of the same effect as changing from f/5.6 to f/11.
And the Mamiya 645 line cameras have focal plane shutters. There are a couple of specialized leaf shutter lenses you can use with them, but most of the systems lenses don't have a shutter.