The same principle applied to us when we were LEOs. You didn't want to have your hand, either hand, have to cross the mid-line of your body to access a piece of vital equipment. Extra mags on the left (for righties), bullets pointing forward - left hand grabs the mag from the bottom of the pistol and pulls it free...on the way down (depending on if it's a tactical reload or a expended magazine reload) the mag is either tucked in the belt or dropped, the left hand then continues to the reload, pulls the full mag from the pouch and shoves it in the waiting pistol mag well of the pistol which is still pointed in the direction of the threat.
