I noticed recently, while riding in an old ge diesel, that when the engineer put the brakes on apply, he tapped the locomotive brake over to the relese position and back twice quickly. How does this affect the train?

And when the engineer puts the train brake on release, when can he put it back in lap, when the brake pipe is at 90 psi or whatever the default pressure is, or does it go by the psi of the equalizing resevoir?