Jean does it both ways. At first she just telekinetically controlled the tennis balls and pushed them towards Null all the way until his power countered hers. With the "Sword of Damocles" training she just held tennis balls over his head so that if he countered her power, he would cause the tennis ball to drop on his head. Jean could also just hurl the balls at him, releasing telekinetic control over them a couple of feet before they hit Null. It provides proof both ways that Null does nullify mutant powers but cannot stop the laws of physics even if they were initially being affected by a mutant power. As we have seen, his power is actually quite selective in that Charles can read his thoughts and send to him (communicate) but if he were to try to mind control him (attack), Null's power would counter it. If Magneto destabilized the magnetic field of Earth completely, Null could not nullify his own demise from the catastrophic damage done to the planet even though Magneto's power was the initial instigator.