Typically, the shooter reaches for their spare mag with the support hand, sweeping it out of the magazine holster either to the rear from the weak side, or from across the front midline in a crossdraw motion. Both positions are extremely fast but the crossdraw position allows access to the magazine from a sitting position, such as in a vehicle or theater seat.
If you are a right handed shooter, you will likely grab your spare mag with your left hand, and thus, use the "Left Side to the Rear" version MagHolder. The Left Hand Magholder is designed to be pulled to the rear on the left side with the left hand, or you can wear it on your right side and pull it to the front in a cross draw with your left hand.
If you are left handed, you will likely grab your spare magazine with your right hand, or the "Right Side to the Rear" version MagHolder. The Right Hand Magholder is designed to be pulled to the rear on the right side with the right hand, or you can wear it on your left side and pull it to the front in a cross draw with your right hand.
See the below images that are helping illustrate the different orientations and how they could be used.