There could be multiple duress passwords, one type would silently wipe the phone while displaying some fake data in a fake/locked down OS. Another would brick the device, another would turn on tracking and call authorities of choice..
Bricking or wiping isn't a good option because it makes it obvious what you did. One of the properties you want from a duress passcode is to convince the attacker (the border force in this case) you gave them what they want. Convincing fake data is hard to generate and easy to spot. Another alternative is for the duress passcode to unlock a subset of data and apps. This lets them see real but uninteresting data like contacts or family photos while keeping your personal documents (business, legal, medical, financial, keychain, or otherwise sensitive) private.
