"What is that round black thing on the back of the holster?"
That round black thing is just a simple rubber puck that we mount to the back of our appendix carry (AIWB) holsters to aid in comfort as well as concealment similar to a foam wedge that you might find on other AIWB holsters. We use the rubber puck because it won't deform or wear out and need to be replaced periodically like a foam wedge. It is also very minimal and easy to install or remove unlike the various methods that are used to attach a foam wedge to the back of a holster (i.e. Velcro, adhesives, etc.).
Comfort
Since it's made of rubber and is round and smooth, it potentially reduces uncomfortable hotspots against your body that might be caused by any irregular or abrupt edges of the holster itself. It also gives you the perception that the holster is only as long as where the puck is mounted to the holster which can dramatically improve your comfort level for larger guns.
Concealment
In addition to the potential comfort aspect, the rubber puck also pushes the bottom of the holster away from your body, which tilts the back of the gun's slide into your body above the belt line. Combined with the concealment wing (aka "claw"), this substantially reduces printing for concealed carry.
There is nothing special or proprietary about this rubber puck. In fact, it can be purchased at some local hardware stores as it is not specific to holsters. We gave this thing a try several years ago after we made our first full-size weapon mounted light holster in an effort to increase the comfort level, and we've found no reason to change to something else as it works very well.
They are now available in 3 different sizes:
The XL size (2.5") is for our full-size weapon mounted light holsters.
The Large size (2") is for our compact weapon mounted light holsters.
The original Standard size (1.5") is for our non-light bearing holsters.
The Standard size can also be used on the back of our mag carriers.
The perceived level of comfort from using the rubber puck is subjective for each person, so if you'd rather not use it or you prefer to install a foam wedge, then the puck is easily removed from the holster with just one screw and threaded post.
Foam Wedges
If you're interested in a foam wedge instead of the rubber puck, we now offer our own foam wedges as an add-on accessory. Our foam wedges are made of 1/2" thick neoprene foam rubber material with a smooth vinyl sheet covering on the face and are available in 1.5", 2", and 2.5" sizes. We can also make them in custom rectangular or square sizes to suit your preference. They attach to the back of the holster with an adhesive strip that holds them firmly in place, but can be removed relatively easy when the need arises. As mentioned before, foam wedges will eventually deform or wear out and need to be replaced periodically, but they do potentially offer a little bit more comfort.
Check out our website for more details and pricing on all of our products!