The most revealing part of the Trump-Xi meeting may come after the cameras leave. The handshake will last seconds. The bargaining could shape the next phase of the world's most consequential rivalry.When Donald Trump and Xi Jinping sit down in Washington on September 24, neither man arrives empty-handed. Trump can threaten tariffs, restrict China's access to American technology and wield the world's dominant financial system. Xi can retaliate through China's vast manufacturing base, its grip on rare earths and its economic relationships with countries Washington wants to pressure.Also read: Five reasons India should watch Trump-Xi handshakeThere is a useful historical…