Rick always found that in heavy air when he is out on the trapeze, holding the mainsheet, uncleated, helps to keep you "glued" to the side of the boat, so it is serving almost as a sissy line.
Even when my main is cleated, I keep it taught in my hand for that same reason.
With regard to sailing when the main has been handed over to crew, the empty hand feels like it's searching for something to hang on to.
