Nationality toggling is the choice of which passport to present so that operative nationality is the citizenship you are treated as holding in a given interaction: at a border, a visa window, a bank, or inside one of your own countries.
If you hold three passports, a third country will usually deal with you as a national of one of them. The document you hand the officer is the usual trigger. Airlines issue the ticket against a passport number. Hotels copy the biodata page. Banks KYC the document you used to open the account. That is the face you put forward. It is not a change of legal status. You still hold the other nationalities. You have chosen which one is operative for that transaction.
Inside a country of your nationality the rule is stricter. The 1930 Hague Convention on Certain Questions Relating to the Conflict of Nationality Laws put it in two articles that still describe state practice. Article 3: each state may treat a multiple national as solely its own. Article 4, the master nationality rule: the other state may not exercise diplomatic protection against a country that also claims you. If you are a British–French dual national in Paris, Britain will not normally intervene with French authorities on your behalf. France can draft you, tax you, or refuse to let you leave under French rules. The second passport does not travel into that room with you. US immigration law has a narrower, operational version. In Matter of Ognibene, the Board of Immigration Appeals held that a dual national who entered the United States as a Canadian could not later claim Italian nationality to switch into E-2 treaty-investor status for that stay. The nationality used at admission stayed the operative one. Consular practice for E visas also looks at which nationality you hold yourself out as. Toggle before you enter, not after.
Travel mechanics are ordinary and easy to get wrong. Enter a country of your nationality on that country’s passport when the law requires it. The United States expects this of US citizens. Leave on the same nationality you used to enter, unless a local rule says otherwise. Book the flight on the passport that matches the visa or the visa-free right you are using. Carry the other passports. Do not put two of them on the same counter at the same time unless the officer asks. None of this is costume. Presenting a nationality you do not hold is fraud. Omitting a nationality on a form that asks for all of them is a false statement.
Wartime and sanctions make the choice sharper. Dual nationals of rival states have always had a reason to pick which face they show in a third country. That is not new. What is new is how often banks and airlines run the same screen. Operative nationality at the border and operative nationality in a compliance file can diverge. A quiet passport at immigration does not erase a US-person flag in a bank.
The portfolio gives you a choice of face. It does not give you a second biography. Toggle on purpose, keep the story consistent with the document in your hand, and do not expect the other citizenship to rescue you once you are standing on soil that already claims you.