Jiggle syphon

The actual term for what is essentially a check valve in a bottom of a pipe appears to be "inertial pump", as searching for that phrase brings up many relevant results, including this one with a good illustration of how it works:

https://www.solinst.com/products/groundwater-samplers/404-in...

These work great! Bought one years ago to empty out our bird feeder in mosquito season: https://www.amazon.com/dp/B000BG1X54/

Only annoyance is that small leaves and twigs get stuck in the ball chamber but it’s easy to clean it with pinkie finger or a stick.