According to Vipassana, all misery comes from "Sankaras". A Sankara is either a craving or an aversion. In other words, wanting something badly, needing it emotionally. The feeling associated with addiction.

You have a lot of different little sankaras going on in your daily life. Vipassana is all about systematically eliminating them. It's a sort of ultra-rehab. But they deal directly with sensations: It doesn't matter what the object of the addiction is. They won't talk with you about cigarettes if you're addicted to smoking. They just teach you to feel yourself all over your body, and whenever you feel a sankara, you have to (gently but firmly) destroy it. You destroy it by simply observing it objectively. Over time, you notice that you have less and less sankaras.

Why is it intensive/hard?

The hard part is having to deal with your sankaras all the time. This is very hard.

The intensive part is because there is so much meditation. It's somewhere around 10 hours a day, if I remember correctly, with the biggest chunk being two hours long. You wake up real early and start meditating right away. You have few hours to sleep. There is no dinner. (This is intentional: it's supposed to be better for meditation.) You aren't even allowed to exercise.

I think it's worth it though. I took it twice.

You can check out their site too: http://www.dhamma.org/