Hitting something with the finger. Usually from being released after build-up tension with the thumb.
Define “proper”. :)
I suppose that thumping something with your finger has less “improper” twang to it, and that smiting something with your finger sounds more awesome still, but guitarists — seldom folks to get hung up on matters of propriety — simply call this finger thwacking.
Is that what you mean?
I confess that if it were me and I were feeling a wee bit naughty, I might be tempted into minting me a nifty neologism like fthwacking for this act, which although perhaps less prim than queenly English would like any good shibboleth serve to separate those who can say fifths and twelfths from those who cannot. :)
The common expression is 'finger flick' as demonstrated on Youtube.