You know those hooks on hiking boots that you use to lace them? Imagine one fastened on your tab in a place where you could press down on it with your thumbnail. Imagine if you could press down on the very edge of it with steady pressure, eventually your thumbnail would slip off. This would be a type of tab sear, which is named after the sear that is a part within a gun that activates by the pressure of your finger to fire the gun. The idea is that you release the arrow when your thumb slips off the sear, to make the trigger a surprise. It's sort of a unique concept, because unlike a gun sear or a clicker on a bow, the tab sear really doesn't have anything to do with the shot activation; it's just a signal to the brain to release the arrow. A grip sear is similar, in that you either put something, or find some irregularity on your grip that you can press against with a thumbnail. This is mainly of interest to dead release shooters, as dynamic release shooters are generally satisfied with some kind of pull through trigger.
I've never tried a tab sear, since I don't like the idea of putting any part of my hand other than my curled fingers under tension. I did try an interval timer for a while to try to improve my hold time. Maybe it worked, because now I'm happy with my hold time and don't use it anymore.
A surprise release is a good idea, like squeezing a rifle trigger until the shot goes off at an unanticipated moment. Many people use a subconscious trigger to achieve a surprise release. However, it is suspected that a subconscious trigger is a major cause of target panic, so people are looking for other ways to achieve a surprise release. Actually, a good surprise release can be achieved by relaxing the fingertips instead of willing them to open, as there is a slight delay between your brain giving the command to the fingertips to relax and the fingertips relaxing enough for the string to flip them open. I've found that maintaining my sight picture as a part of my followthrough while my relaxed fingertips are flipped out of the way seems to be a good solution to this problem (easier said than done, but that's what I'm working on now).