Oh, sorry guys. The guy who sold me the camera told me that it is actually the other way round. Here is his email:
Hi Nicolas,
The diaphragm hold button is indeed the EE lock, which freezes the aperture at the current exposure. The Bolex 350 is set to recognise 40D/60T so compensate with 100D and maybe hold the lock on the sky, as in bright/snowy conditions it is very easy to overexpose with 100D. Though that can be a nice effect in itself. Also be sure to TAP THE CARTRIDGE before insertion (they jam) and be sure to insert the filter key, as 100D is balanced for daylight so you need to disengage the internal 85 filter.
Ideally, in such snowy conditions you would take a cheap spot meter with you and set the aperture as you shoot but it could well prove quite tricky to do that. I would just try to carefully gauge the exposure by looking through the viewfinder and adjust each shot accordingly.
Just remember your 100D stock is faster than what the camera automatically exposes for, so you need to slightly close the aperture for the correct results. If this all proves to be a little too complicated (will you be running & gunning handheld or taking static shots?) then maybe just permanently compensate the exposure on the camera and then point out to the processing lab technician that it is possibly all overexposed. The flaw with that, is by shooting in snow you might get a white glowing screen and slightly blurred details. It all depends on just how bright it is.
I do hope you get great results!
Best wishes
Esta