Peripheral event system
A routing matrix inside a microcontroller that lets one peripheral's event trigger another peripheral's task directly, without waking the CPU.
The traditional way to have a timer kick off an ADC conversion is an interrupt: the timer fires, the core wakes, the handler writes to the ADC's start register. That costs power, microseconds and jitter. An event system runs a wire between the two in hardware instead. Atmel built an Event System into XMEGA and later the SAM D family. Nordic calls its version PPI, Programmable Peripheral Interconnect, with a distributed variant, DPPI, in the nRF53 and nRF91.
The model is simple: each peripheral exposes events ("timer reached compare") and tasks ("start sampling"), and a channel connects one to the other. The result is a kind of circuit board described in firmware. It is brilliant for low-power designs, and equally effective at producing behaviour nobody can find by reading the C code.