Talking to someone during a live gig is not recommended, lest you want to end the night with a sore throat and a hoarse voice. But you're out, enjoying yourself and socialising is not only desirable, but inevitable. What if it were possible to avoid the aching vocal cords, yet still get the message across?
The theory is that by covering the ear with the tragus, you block out much of the ambient sound, making it much easier for the person to hear you. From the Reddit post:
You would use your thumb to push in on the person's tragus and speak into that same ear. You still have to move your mouth semi-close to their ear, but blocking the ear filters out the really loud noise and allows more of your own voice to get in.
This is not the sort of thing you'd try with a stranger — moving your hand to block someone's ear is not the best way to make friends. However, your partner or someone aware of what you're trying to do would certainly be more accommodating.