I'm pretty sure abduct (hook) is just a targetted spell. Click on the target unit, and like with all other sc2 spells, if the unit is not in range he will try to move into range to cast the spell. Depending on the range of abduct when used against colossus and siege tanks, it will probably be difficult to do without getting your viper sniped, so it doesn't sound like it will be too unstoppable.
Except blizzard in their wisdom made it fly, so colossus and siege tanks can't hit it, which makes it infinitely better than neural parasite already.
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