How does the automatic change in process state from "Running" to "Ready" work?

A process can change its state from "Running" to "Ready" in one of 3 ways:

1. Automatically
2. Explicitly - the process voluntarily gives up the processor.
3. Implicitly - process triggers system blocking calls and must wait until it is processed (e.g. I/O).

I understand 2 and 3, but how does 1 work?