Lying Finger Cross
Sometimes when a character is forced to make a vow, oath, promise, gesture, etc. they don’t actually mean, they will cross their index and middle fingers in a way hidden to the character they’re talking to but visible to the audience, to show they’re actually lying. Most often the person will have their crossed fingers held behind their back, but off to the side, under a table, or somewhere else hidden is also possible.
Truth in Television, of course, which is the reason why it works as a visual cue, often in the form of An Insert. There’s a number of guesses out there as to why it became a common gesture for lying, but the general idea is that it somehow either protects you from the consequences of lying or absolves you from being held to your word to begin with.