As I recall, there are photos of some of his earlier versions of the tab sear buried somewhere in the many pages of this topic. These appeared very similar to the lacing hooks that are on some boots once you lace past the eyelets.
His more recent versions appear to be two nylon washers, one slightly larger than the other. The larger one goes on top. In either case, they are fastened to the tab with a rivet or some similar way.
The idea is to hook your thumbnail on the edge of the sear, and press down on it as you expand. So you have to select some place on your tab to mount the sear where you can press down without having to move your thumb to some unnatural position. When your thumbnail slips off the edge of the sear, that is your trigger to shoot the bow.
For this to work, the trigger has to be a surprise. Obviously, you could just force your thumbnail down on the sear and make it slip off, but that would defeat the purpose of the sear. You have to steadily increase pressure until your thumbnail slips off at some unknown moment.
In fact, the key to Joel's whole system is to focus all your attention during expansion on the movement required to activate the trigger. When Joel asks you the question, "What were you thinking about during the shot?" any answer other than focusing on the movement required to activate the trigger is the wrong answer.
An alternative to the tab sear is the grip sear, where you mount the sear somewhere on your bow handle where it can be conveniently reached by your bow hand thumbnail.
Joel encourages using your own imagination in creating a sear, or even using some feature, such as a ridge or bump on the bow handle, that is already there.