Martin's "Hover" button on the app. It resets his altitude to 3 feet above the ground ten times a second. The book describes it as a very disorienting, shaky affair, which Martin compares to standing in a bus that has no shock absorbers. The thing is, he's not being thrown up and then falling down. Instead, he's teleporting to the same location multiple times a second, and teleportation is already established as having no physical effect (i.e. one moment you're somewhere, the next, you're elsewhere). Sure, he'd feel gravity pull him, but the pull would be negligible, and he probably wouldn't even feel it over a tenth of a second.
