What is the box trick in math?

Could you elaborate on what you mean by the "box trick" in math? Is it a specific problem-solving technique, a mnemonic device, or something else entirely? I'm curious to know if it's a commonly recognized method in mathematics education or if it's a more niche approach. Additionally, if you could provide a brief example or explanation of how the box trick works, it would be greatly appreciated. Thank you for your question!