Best scene in the movie. Joker's speech about desire becoming surrender becoming power is an interesting window into his mind and how he views the world. I think the Joker sees allowing yourself to want to surrender to the insane contempt he has for the world is, from his viewpoint, a liberating experience. The power one gains from surrendering your sanity is that you're no longer constrained by the rules. You're now free to do as you please, consequences by damned. He thinks his insanity is power in and of itself. An interesting aside: in the movie novelization Joker originally planned to walk away and let Harleen die in the vat of chemicals. He viewed her as a useful tool that had become a nuisance. However, for some explained reason he changed his mind and begrudgingly jumped in to save her - she can't swim, after all. That's why he grunts in irritation just before he leaps. Some have suggested he does so out of some inner turmoil, i.e., an inkling of love in his dark heart. I'm betting even the Joker doesn't know why he did it.