Tosca's initial reaction is to stop dead in place. It's quite safe, but hard to actually go anywhere.
I've been riding an endurance saddle with wonderful EZ Ride leather-covered stirrups, after 40+ years of riding in English stirrup irons. So the other day, I put a dressage saddle on her, mounted and couldn't pick up my right stirrup. I was thinking "Good grief, I can't believe I can't pick up a stirrup. I am on this really green horse with only one stirrup. I am going to get really hurt." Understanding here that the mare was standing stock still. Finally, I leaned over and guided my foot into the stirrup. You just have to appreciate that phlegmatic nature - hers, not mine. Nancy