Many people that carry concealed do not want to have all of the bulk that goes along with everything that you can put on your belt. Carrying a spare magazine is always prudent though, allowing you to engage a threat and top off your pistol in the event that there might be more threats in the area.
The SnagMag is a product that holds a magazine on your support side pocket. Looking just like a knife clip, it conceals your magazine without making your belt look more like a full duty belt. I am sure that drawing the magazine efficiently would take some practice, but the idea is sound. Many people I know that carry daily tend to just throw a spare mag in their pocket. This becomes a problem in indexing the magazine as is can be pointing just about any direction. The SnagMag looks to eliminate this problem by making sure the magazine is where you need it to be for proper indexing.
Here is a video from Ultimate Concealed Carry explaining the SnagMag. If you want more info on the SnagMag you can check out there site at http://www.snagmag.com/