The mobile holder comprises two parts, the "belt clip" and the "base" (not shown by Amazon). You replace the battery cover that came with your phone, with the belt clip as illustrated above. You then attach the base to, for example, the dash of your car (4 screws are provided, you could also use double-sided foam tape). When you're in your car, you take the phone off your belt and slide the clip through the slot in the base.

The clip holds the phone quite well to the base. It won't fly off unintentionally while driving, yet is easy to clip in/take out as desired. But the clip is a bit too short and slippery to hold it securely to your belt. It's not unworkable, but it is less stable than, say, Body Glove's swivel clip.

This holder inexpensviely gives you a secure, unintrusive place to put your phone in your car, while also providing a belt clip. So there's a trade-off between it's utility in your car and on your belt.