As with any minion's abilities, the warlock can set Auto-cast on or off for Whiplash. Because of the lengthy cooldown (25 sec.), it's probably better to have Auto-cast turned off, letting the player determine the best time to cast it.
Patch changes
Patch 4.1.0 (26-Apr-2011): Whiplash (Succubus) now knocks targets away from the center of the targeted location, instead of away from the Succubus.