What the hell is an AFAK? Easy, it's an "ankle first aid kit." The next question is why would you wear one? And that depends. You don't really think about first aid items--until you need them, and usually by then it is too late. If you are a first responder, this is an excellent way to always have a couple of basic items on hand for yourself in an unobtrusive manner. You won't even be aware you are wearing it. If you are the average slightly cautious/paranoid citizen (like me), this is an excellent means to always have emergency medical gear on your person.
I tested the Ryker Nylon AFAK this past week. Jogging and jumping had almost no effect on my awareness that it was even there. Skinny jeans are a no-go, but regular pants fit neatly over the kit. Ryker Nylon also sells its own accompanying medical gear, or you can simply use your own. The Ryker Nylon carrier is about $80 and the medical items are sold separately. In this example I have a tourniquet, a traumatic wound bandage, scissors for cutting through clothes, and rubber gloves (I forgot to mention the gloves in the video.).
Overall, I highly recommend this kit. It is comfortable and you can engage in all of your normal movements to include running and jumping--without being aware that you are even wearing it. You can find it at https://www.rykernylongear.com.