The only thing I can think of with Colin Hanks chaining himself up is this:
Gellar is a split personality (or a Dark Passenger). Hanks chained himself up before Gellar took over the body, and only Hanks knows where the key is or something. He can't unchain himself until he's back in control, and thus his Gellar personality can't make him do the stuff that Hanks doesn't want to do.
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http://img20.imageshack.us/img20/6782/timj1.png