The answer turns out to be “a lot of things”, and they all happen very quickly, and it doesn’t end well for the batter (or the pitcher). I sat down with some physics books, a Nolan Ryan action figure, and a bunch of videotapes of nuclear tests and tried to sort it all out. What follows is my best guess at a nanosecond-by-nanosecond portrait:

Damon Runyon via the El Paso Herald, December 20, 1912:

Ferd - that’s a name - Schupp (which is also a tag-line), Ferd Schupp is a new left-handed pitcher who was dug up for McGraw last season out of the Wisconsin-Illinois league, is said to have solved the problem which has been studied by pitchers for 50 years—how to throw a rising curve.
...
[Schupp] is said to have a real rising curve—a curve that changes from a straight ball to a perceptible arc upward.

We go now to our science ...