Can be used after you Life Points are 3000 or below. Play the Field Spell "Skyscraper". If there already is Field Spell on your side of the Field, Return it to your hand. This skill can only be used once per duel.
I just tried to use this skill in the battle phase after lowering my LP by attacking with a weaker monster, but it turns out you can't activate this skill in the battle phase. What a shame, it would have been a really good clutch win, too. I think it's worth considering letting players activate skills proactively like this, waiting a turn to get damaged and also survive it is often simply not an option.