The mechanism: Buried in Presidential Proclamation 10984 (October 2025) is a release valve nobody priced correctly. Commerce can cut Section 232 steel and aluminum tariffs on Mexican and Canadian metal by up to 50% — but only down to a 25% floor, only for USMCA-qualifying content, and only for producers who commit to matching new U.S. primary production capacity. Commerce opened the application window April 23, 2026. That valve can also close. If Commerce slow-walks approvals, caps the relief pool, or a future trade fight scraps the adjustment mechanism entirely, Mexican-melted steel and aluminum snap back to the full 50% rate with no USMCA discount. Wall Street's reflex is "tariffs = bullish domestic steel, bullish Detroit's pricing power." That's half right. It ignores that Detroit's own supply chains run through the exact plants a steel-tariff shock would tax.