hint: leverage
Is it this?
Nobody is impressed by what you can't do. -THJ
Hmmm... Interesting... It appears to be some sort of thumb / leaver combination. A thumb-jack? For really fat, lazy thumbs? You place it on the top of your support hand and when trying to obtain the correct handgun grip you adjust this "thumb-leaver" to raise or boost the fat lazy thumb into correct position. It's all making sense now.
It's like a thumb on a riddle, wrapped in a mystery, inside an enigma