Seems like everything these days regarding how you setup and use your gun involves more opinion than science. And when it comes to whether you face your mags forward or backward, many expert opinions have been lobbed by both sides. But nobody seems to talk about the mechanics of this practice.
Typically a reload requires your index finger - regardless of left or right handedness - to be placed around or on the front of your magazine; the side where the bullets are facing. Whether you holster your bullets facing the front or back, this grip is generally deployed the same way. The theory here is by placing the index finger on the front of your magazine, during insertion you simply have to slide it into your grip to be ready for the next shot.
But for front-facing mag shooters, there seems be a lot of twisting of the hand required to complete this process. More motion leaves more opportunity for failure.
For front-facing mag shooters, as you release your support hand from the pistol and reach down to grab the spare mag, your hand will rotate from a palm facing your body to a palm facing your rear; you must twist it 90° in order to get your index finger on the front of the mag.
Once you pull the mag from the holster, you must untwist your wrist back 180° so you can insert the magazine into the magwell. The point of all this is to ensure your index finger is riding on the front of the mag the whole time. You use that finger to align your hand with the front of the magwell, increasing the accuracy of a speedy reload.
This technique is encouraged by most instructors because this is how they learned to holster and reload mags. But it’s not the only way. In fact, I feel it is both a highly inefficient method and leaves a lot of room for error.
REAR FACING MAGS
As you release your support hand from the pistol and reach down to grab the spare mag, your hand will move directly from the gun to the holster in the very same position, palm facing the body. But this time, no twisting is required. Your hand is in a natural position and stays that way as you grab the magazine now with the thumb on the primer side of the bullets, the back of the magazine, and your index finger wrapped around the front of the magazine.
What we have essentially done here is eliminate the index finger from the “indexing” of the magazine front. Instead, we are going to use the thumb. A much stronger and shorter part of the hand to perform the same function as the index finger, also with a lot less twisting.
Next, once you extract the mag, you raise it to the magwell keeping your palm still facing the body, still, in it's natural state. As you insert the magazine into the magwell, however, notice you now seem to be aiming it with your thumb instead of your index finger. Your index finger should not be pointing but wrapped around the magazine, which also increases your grip on the mag.
Indexing the magazine with your thumb instead of your index finger also allows you to execute a faster transition back into the grip. Indexing with your finger requires you to twist your hand back around the grip during insertion. There is no twisting motion required with a rear-facing grip. Your thumb indexes the magazine and simply slides into position on the grip after insertion.
The key difference between these two techniques comes down to the hand twisting required for front-facing mags. In my practice, eliminating this action is simply an efficiency of motion and reduces the opportunity for error. By facing my mags to the rear, I am able to perform the reload without twisting my palm at all, and execute a reload just as fast if not faster than the alternative.