Thadapters are basically a spring loaded plug that takes place of a standard feed plug.
Picture history of the thadapter as far as I understand it from reading MCB:
First made by Thad aka the weltman on his original lever action (the lower one, I think the first lever action sheridan ever made), no spring behind the thadapter plug (made around 1992??):
Then spread to bretg's KLs (the first production lever actions, pictured is an integrated one):
Add-on versions were made as well:
I have 3 of them, I really like them, they load a tiny bit slower than just dropping the balls straight down a side tube, something about the balls making a 90* turn rather than rolling straight into the feed tube from the 10-round tube. But the benefits far outweigh that minor drawback, aka no fumbling around with feed plugs.