How does a USB keyboard work?

Caption notes

Ben Eater contrasts how PS/2 and USB keyboards send keystroke data. A PS/2 keyboard can send data after a keypress. In the USB exchange he describes, the host polls first and the keyboard replies, so even with fast transmission a keystroke may wait until the next poll. He also explains how the USB signal carries both data and timing information. In the NRZI encoding used in the demonstration, a transition represents zero and no transition represents one, while bit stuffing keeps the signal from going too long without a transition.