Here's what the Thai government has to say in an official press release concerning the treatment of elephants in their country: "By nature elephants are temperamental, disobedient, and sometimes they can be violent. A mahout carries a hook with him when he trains an elephant. From the start of elephant training an elephant learns that a hook can cause pain. In due course a hook becomes a mere symbolic gesture for obedience from elephants, and mahouts have no need for their actual use..."
So, in other words, they are used to inflict pain on the elephant when they are typically taken from the mother (usually around age 3 when they stop suckling), then the mere threat of the tool is enough to curb some behaviors. Kind of like how a puppy who has been beaten will shrink away from you as you reach out to pet him. Merek, I'm sure these tools don't raise an eyebrow where they're used, but that doesn't make their use any better. The difference is that carpenters don't use hammers to beat up animals, generally. Unless there is no difference between a piece of wood and an elephant, then the anology doesn't really work.
For the record, I'm not accusing Santi of being a jerk or an animal abuser, etc, nor am I trying to say everyone in a certain country is guilty of this behavior, but if you travel to other countries you'll see some pretty appalling behavior inflicted on animals and the locals don't even give it a second thought. That doesn't make it any better or justify it. We live in a very insulated world in Western society and I understand that, but that doesn't mean it's right to beat animals to subject them to do the work we want them to do simply because we can and, heck, they're there so why not? I'm sure there are ethical ways to treat "work animals" and hopefully that's what is more normal in places like Thailand, but you can find out plenty of information about the treatment of elephants throughout Asia, as well as what these hooks can be used for (if you have a stomach for the pictures and videos), if you're so inclined.