Sidearm partial reload detail

I was just watching a video from StoneMountain where Hé was playing as Recon with a sniper rifle during the Multiplayer event.

At some point he reloaded his sidearm, which was a partial reload since his magazine wasn’t empty. I noticed an interesting detail where the character holds the magazine, releases the magazine from the sidearm so that he holds both magazines in one hand and then inserts the new magazine. 

I’ve never seen something like that, at least not that I can remember. I was just fascinated by it, I don’t even know if it’s actually practical in real life.