There are two types of reloads in a fight.
Speed reload and tactical reload. When executing a speed reload you eject the empty mag while inserting the fresh/loaded mag. In a good speed reload the two magazines pass each other (one headed for the ground and the other in your non-shooting hand headed for the magazine well). You go back and pick the magazine up later when the immediate fight is over and your life is not in immediate peril.
A tactical reload is when you have a few seconds to pull out your partially empty magazine and insert a fully loaded one. In that case you put a fully loaded mag in your not shooting hand, with that hand, pull out the partially empty mag and insert the full one, then insert the partially loaded or empty mag in a carrier or a drop bag on your belt(if you are carrying one).
Usage: Speed reload...I'm shooting and need to continue to engage RIGHT NOW.
Tactical reload: I'm not shooting and have a little time to get my stuff together for the next engagement.