How does unidirectional breathing in birds and reptiles work?



I’ve looked this up and all the sources I can find don’t seem to actually explain the structural difference of unidirectional from bi-directional breathing. I can’t wrap my head around it, even with diagrams.

In: Biology

Basically it works like this.

When a bird breathes in, it fills up its lungs, and also fills up two reserve balloons/sacs.

Then, when the bird breathes out, the air that filled up the reserve sacs gets pushed through the lungs. Meaning the lungs are getting “fresh” air on both inhales and exhales.