I think the idea is you try to play it when your health is low, destroy it when your health is approaching 20.

How so?

Start with 20 life, get obliterated until down to around 1, now you’ve got another 18 life to use. And maybe there’s a way to reverse it to get another 18?

I guess this can be combined with some card that lets you enchant another player instead of yourself? If that exists?

If your life total is 20 or more and you control an effect that states “you can’t lose the game”, then an infinite loop is formed. If neither player wants to end the loop by removing one of these two cards from the battlefield, the game ends in a draw.

It's a Self-Destruct Button!