Realizing the gravity of the situation and the horrible mistake he’s just made, he cuts off power to the spell. To his increasing horror, you flex the true extent of your arcane vampire skills and fight to keep the magic connection open for longer, forcing the spell to continue casting and managing to both maintain it and continue draining magic for a whole ten seconds before you lose your ‘grasp’ on it and the staff finally stops channelling. His half-panicked flailing of the staff does nothing to help or harm the process, but it was very amusing to witness all the same.