Right. His ult just deals damage to everything around him for a few seconds after the channeling finishes. He can move, attack, cast, whatever he wants while it's active. So generally the goal is to land as close to the center of the enemy team as possible (for maximum damage) and immediately cast his silence to bounce around the group and disrupt their ability to attack/escape, fear the highest priority target and start draining whoever he can (or, alternately, activate Zhonya's to not die/get CC'd himself).