In theory, your swing may bottom out in line with where your left armpit is at AT IMPACT. Please note that this may not be precisely the same location of the left armpit AT ADDRESS. Another factor, in some swings, is that the left shoulder may actually be moving upward, to some degree, just before and during impact, so the lowest point of the swing may actually occur behind the left armpit, more to the center of the body.

The arm swing may be centered on the left armpit but that center is also moving up and down, in and out, forward and backward.. The golf swing is a dynamic series of events.