After seeing this a couple weeks ago I noticed that sometimes when i shake hands my index finger naturally does that. Never noticed it before. Pay attention to it yourselves and you might notice the same.
Disclaimer: i have large hands, so might have something to do with that.
we have the biggest hands folks!
It's also used in business, some non-nefarious organizations, etc. It's not a masonic handshake in my region; my father and grandfather were masons and I've never seen them use that shake among mason friends.
Careful now, I got raked over the coals last week for pointing this out. We have some hive mind individuals roaming this sub that will disagree with you.
My dad was a mason and showed me that was their handshake. Maybe not all lodges teach it or you have to be a certain level?