This is something that really sucks for many of us, so I did a quick experiment with pulse oximeter. I wore the oximeter on my right hand, and raised my left arm overhead for several minutes. Heart rate quickly dropped, followed by an oxygen drop. Same thing if I wear the pulse oximeter on my right hand and raise my right arm. So in my case, at least, raising an arm above my head somehow triggers my heart rate to slow down, which leads to decreased oxygen saturation. And then the heart rate stays somewhat low instead of rising in reaction to the decreased oxygen.