This time around he’s explaining H-Bridge motor controllers and how they work! Even if you don’t have much (or any) experience with basic electrical circuits, [Afroninja] explains the concept ...

The on period, known as pulse width, is modulated (a.k.a. changed) via the code, allowing you to control the value sent to your components. PWM pins work the same way across all Arduino boards ...

This KTB2140 current-mode PWM controller can operate as an active ... Protection features include OCP, short-circuit, thermal, under-voltage lockout (UVLO) and over-voltage protection (OVP).

Of course, the standard way of controlling CPU fans these days is with PWM, so he built a circuit which essentially converts the PWM signal from the motherboard into a phantom thermistor.