I am sure many of you have come across this trope in some anime/manga, where a character gives a thumbs up with an imprinted spiral.
The spiral is probably a simplified hanamaru:
which basically has the same meaning as a thumbs up, or "good job".
The focus of this question, however, is on the spiral that appears on the index finger.
In the two examples shown above, the characters were obviously not showing approval, so the idea of the spiral being a simplified hanamaru is out.
Someone has asked the same question here:
Spirals. Just spirals. Apparently there are "spirals of depression" and "spirals of happiness?" I think I heard the depression spirals are meant to represent "sad spirits/ghosts" or something like that? But if the character has spirals on her cheeks, she's happy. If there's a spiral on her finger when she points at you, she's being triumphant or prideful. Can anyone clear this up? :3
Is there any source to that statement in bold?
My question is: what is the intended effect of the spiral on a character's index finger, and where did this trope or art style originate from?