Hot air attracts cooler air. So imagine a tornado swirling in the attic of hot air grabbing up cool air. This swirling motion isn't vertical though, it's horizontal. The vortex rolls over at the highest and hottest point of the attic just under the rafters. This allows the cooler air lower towards the attic floor to be swooped up slowly. As this hot air reaches a certian temperature, it will locate the gable vent and exit. The path of least resistance.

https://www.onyxxteriors.com/QA/Do-gable-vents-let-air-in-or-out.php