You have never had your bank account frozen/garnished have you....that "piece of paper" can do both very quickly
If you showed up at my bank with a court order from Japan they aren't going to touch my account. The second they improperly touch funds in my account they open themselves up to a lawsuit (and I'm sure their insurance company wouldn't like that). The only way they are going to freeze or garnish my account is if they are legally obligated to and court orders from other jurisdictions are not generally legally binding.
They wouldn't show up with a court order from Japan. They'd show up in court here with the foreign judgment, and depending on the jurisdiction, have the foreign judgment domesticated and enforced, obtaining an enforcement order from the local court. Then they would serve the bank with an order from the local court incorporating the judgment of the foreign court.
Japan is, of course, party to the treaties which allow this, and United States law has, itself, recognized the judgments of foreign courts both in common law and under statutory law passed pursuant to the mostly Twentieth Century treaties.