Despite the chain's length, it is treated as a normal reach weapon (doubling a creature's natural reach). The twenty feet is for when it is used as a grappling hook. The combat hook's length for this purpose can be increased for a cost of 50 gp per ten feet, to a maximum of 50 feet. It's combat reach does not change. The combat hook is an exotic two-handed weapon. Weapon finesse can be aplied to its usage. The Combat Hook can be used to make trip attempts. If you are tripped during your own trip attempt, you can drop the hook to avoid being tripped.