@kenshirriff the "upside down" p-FET only lets the charge in the donor transistor through, and chokes off before the clock voltage can start contributing. Here's a quick and ugly Falstad sketch I made on my phone:
tinyurl.com/28yhhbq4

You effectively get a constant 0.7V offset from the MOSFET threshold added to it because of my sloppy circuit, but I imagine the real one accounts for this at the input/output.