I'm reading "Fate is the Hunter" and just read something interesting. Gann
states that they would climb to 100 feet above their cruising altitude and
then descend the 100 feet back down. He called this "flying on the step."
He claims that it bought them a few more knots of airspeed.

Has anyone heard of this? Is it normal practice? Or is it one of those
practices that have been disproven?

Personally, I've never heard of this practice.