In my experience, starting exercise might actually trigger an attack, but if I just push through after 10 or 15 minutes and I really warm up it actually reverse and my fingers are warm, even in cold conditions. This is only an issue during colder weather, so my theory is that my fingers are on the verge of going white, and anything might trigger an attach. Exercise usually involves being outside, and being dressed more lightly than normal. Also, maybe more muscle tension is present during exercise. So the fingers spasm up like normal and get numb and white. But once the core temp starts overheating with heavy effort, the body eventually figures out that it needs to radiate out some extra heat, and it opens up the blood vessels again and everything is good. The trick is to know yourself really well so that you can recognize when you will reverse the attach with effort, or when it’s time to give up and go inside somewhere warm.