You can't lift someone up if they're touching your chin. Even if you're a lot larger than them.
The secret doesn't actually have anything to do with the chin. It's because when someone is touching your chin, they're creating more distance between the two of you.
When you lift something up that's heavy, you need to hold it closer to your body to get leverage. So when someone's touching your chin, they're creating at least an arm's length of distance between you.
Which makes it nearly impossible for you to pick them up.