Even with the distraction of the punching bag coming back at he, not losing her wand with the disarming spell was basic. Still, there was some merit in pretending to slip up, let the other person think for only a second you were fumbling with your wand. It was the sort of play where she would make sure to have a spare wand on her person - able to reach for that one while the attacker thought they had won until it was too late.
However, this morning her spare wand wasn't within reach. So she choreographed a sort of fumble that allowed her to kick the stool that previously held her water bottle toward him. As the stool went flying she readjusted the grip on her wand and sent a blasting spell toward the punching bag to have it explode between them.