Originally Posted by moxfyre

Actually, you cannot use a chainwhip to remove a modern freewheel (or even this one from 1985). The tool engages the freewheel body in such a way that the cogs do not need to be immobilized from coasting, unlike with a cassette. What is critical is having a good grip on the tool, since you're basically unscrewing a giant stuck screw... hence the vise or giant wrench (which clamps onto the tool, not the cogs).

Follow the directions at http://parktool.com/repair/readhowto.asp?id=48, in the section on "Threaded Freewheel Removal and Installation".

There seems to be a lot of confusion between freewheels and cassettes in this thread...