I didn't intend to reply again on this matter, but since someone else is now having a go at me I suppose I have to. Hopefully this will be the last time. Once again, if George wishes to have the last word he is welcome to it.
George, Stroebel can say what he likes. 'M' synchronization occurs before the fully open position because bulbs took a relatively long time to build up to their maximum output, but electronic flash is much, much faster. Conventional leaf shutters are designed to trigger electronic flash at the point where the shutter blades reach the fully open position. (If you really don't believe me I suggest you dismantle a few sometime - you will find it very instructive, and provided you take care with it it is really not as difficult as many suppose.)
If you want to be really picky it is just possible that the circuit might be closed a minute fraction before, but certainly not so as the blades are still partially covering the aperture (even at its maximum) - so the shutter is, in every meaningful sense, fully open.
If this were not the case low power flash bursts (which may be effectively over within a few 1/100000ths of a second) would be spent before the shutter had fully opened. As it is though, any synchronized shutter which is working normally will be perfectly okay with the shortest of flashes even at their typical highest speeds of 1/500th.