(wheel squeek!)
In the case of the Hayden tranny cooler, it's the built-in turbulators: you'd never know it without looking it up.
A heatercore is a turbulator by design, and it's quite visible, if you look at one close up, and that's a hint: the airflow brushes up right against it too.
High surface area (inside and out), turbulent flow, large volume of air going through... That's what makes a heatercore so good.
You can find out more from Bill's
Radiator Roundup