I won´t to it cause its way to unhealthy
there is bacteria inside your lungs and mouth changing the toxins of the smoke into even worse ( way worse) compunds ( newest studies about passive smoking, thats why its so dangerous)
so you exhale those compounds in the bag you are going to inhale again

holding in the smoke for some time is real good, cause the lung has to absorp the thc from the vapours and that progress takes it time , otherwise you let some real high % out again