The soc on the original Black had two 200MHz PRUs which ran independently of the main processor and could be used for signalling and sensing tasks that require tight timing guarantees, and can then trigger interrupts on the main CPU.
The PRUs are simple single-issue in-order CPUs designed to make cycle-counting easy. They are more than an accidentally turing complete useful combination of a down-counter and shift-register. Don't get me wrong the RP2040 PIO is a very flexible, powerful peripheral for a sub-$1 MCU. A single PRU is probably larger than the whole RP2040.
The soc on the original Black had two 200MHz PRUs which ran independently of the main processor and could be used for signalling and sensing tasks that require tight timing guarantees, and can then trigger interrupts on the main CPU.