Well, an actual animation of the BB cup in a slightly larger shell, would be better. HAH, here it is:


EDIT: This pictured example would represent the non-drive (left) side of the bottom bracket; The pedaling force forward is to the left, counter-clockwise, and thus same is the closest contact point between the cup (blue) and the shell (red) due to radial load at the bottom bracket, indicated by the green arrow. But then notice how the precession causes the cup to slowly rotate opposite, clockwise, to the right, thus the left side of the BB requires right hand thread to keep tight. The drive (right) side is opposite, precession to the left, so requires left hand thread.