Yes, I am American, from California, and I use/hear "when the hammer falls" all of the time. We also say, "when the pendulum swings" which is the same. These phrases are used to state that there will be adverse consequences for a person who didn't heed our advise, i.e., do what we suggested.

When the hammer falls comes from auctions when the auctioneer bangs his gavel/hammer to end the bidding on an item so when we say at hammer fall we mean at the end. The game was very intense with the lead switching backward and forwards several times in the closing quarter but at hammer fall, team A had a narrow advantage.

There's a Scottish band called Pallas who date from the 1980s. They have a song called 'When the Hammer Falls'. A great song but it only has 25 views on Youtube!

I want to second what Connor Bost said: "Bring down the hammer" is a pretty common idiom. "When the hammer falls" makes sense as being related to it, but it is rarely used. I might have never used if. PS: I do want to disagree re: "waiting for the other shoe to drop." It literally refers to when you hear someone upstairs getting undressed, and you hear one shoe hit the floor, you know that the other will hit the floor soon. So it is used when one thing happens, and that tells you that something else is going to happen; "the other shoe is about to drop." So, if your boss tells you to come in her office, that usually means you are in trouble. So, if she says, "first, I want to thank you for bringing donuts to the office this morning," you know that she is going to say something else: She did not call you in to the office just to say that! So, after she says that, you are "waiting for the other shoe to drop": you know something else is going to happen (or be said) very soon

Hi Sasha, I’ve not really run into this exact phrasing before. But I can offer a couple of sayings that might be similar and more commonly used. “When the other shoe drops” This is a saying meaning when someone has to face the predictable consequences of their actions. E.g. You might have fun getting into fights now while you’re winning, but one day the other shoe is going to drop. The person here is telling you that at some point you’re going to lose a fight and face the consequences of that behavior. “Bring down the hammer” This is a different expression that means someone or some group is bringing severe punishment for some bad behavior. E.g. When the boss finds out you were late every day this week, he’s going to bring down the hammer. Similar to the first saying, but usually the speaker knows who is delivering the punishment. These are my thoughts, hope that helps.