The hand signal just needs to be silent, so it can be flashed secretly at strangers. In this case, it worked. The teenager was able to flash the signal at a driver without alerting her captor
It worked this time because we're in a local optimum of sorts where enough community members understand the signals to help and enough attackers don't know about the signal to prevent it from being made. However, as community consciousness of these signals grows (through articles such as this) attackers will get wise to the practice. Hence, they're self-defeating.
I'm sure in a restricted set of circumstance this may help people (as it did in this story), but I sincerely doubt it'd lead to a material increase in kidnapped people being liberated.
It'd certainly be less effective than other interventions such as cracking down on people trafficking networks or poverty reduction programmes.
The expressiveness of hand gestures (in general) is quite limited and any gesture used for covert communication must make a tradeoff between conspicuousness to the public and conspicuousness to the attacker. This limitation restricts the situations in which they can be made and results in gestures that could be easily dismissed or otherwise misunderstood and thus limit their utility.
Clearly you've not met many sign language speakers. Or Italians.
This is a perfectly cromulent covert hand gesture. It has high coding entropy - I've literally never once seen someone make this gesture (outside of videos demonstrating it). Fingerspelling is not remotely similar since it's a sequence of codes in rapid succession.
Sure was material to this chick
* kidnapper distracted by other things
* kidnapper distracted by having been on the road long enough to be spending all their energy on "not falling asleep" and "paying attention to the road ahead"
* victim in back seat, partially hidden
* kidnapper has temporarily left victim alone in a place they feel confident flight is unlikely
If it starts to become too popular then once stories start spreading of victims who managed to escape despite their kidnapper noticing them using this sign, then new signs will be suggested and circulated. Asking a bartender/waitress if they can make a particular (nonexistant) drink. Blinking in a certain cadence. Crossing your arms and tapping your upper arm five times. Whatever. Language evolves.
I've been in the women's bathroom in bars and seen signs posted out of view from outside that say "if you need help escaping from the person you are with, say this innocuous but specific phrase to the staff". This was long before TikTok existed, and maybe before this gesture existed - I dunno, maybe I just never got shown it.
With regards to the bar examples, they may work in a constrained context where the victim is familiar with the venue. They wouldn't work in the case of victim being transported around under the control of their attacker.
There is a sweet spot between "enough people know what this signal means for it to be worth using" and "so many people know this that my adversary knows it too". The signal this kid used may be about to cross the upper threshold soon, but it worked for her.
Surreptitious and obscured communication channels have been being created and getting detected for most of human history. Look at the continuing cycle of teenage subcultures creating slang to perform the dual function of marking a member of the group and obscuring one's plans from one's parents; look at Thieves' Cant; look at histories and manuals of spycraft. And those spycraft books will probably have suggestions on what to do if you're in a situation where signals just aren't viable, too; signals are one tool.
Yes, the attacker can strip-search the victim for smartphones and GPS trackers and then tie them down in the trunk, but the game is getting harder for kidnappers.