NoOb Question: What's a chain whip for???

[8200rpm]

I bought a tool kit and it has a chain whip. What the heck is this contraption for???

[artmichalek]

It's used to hold the cassette in place when unscrewing the lock ring to get it off of the hub.

[mattjf]

Heres a picture from the park tool website showing it being used:
http://www.parktool.com/images_inc/repair_help/FR5andSR1use.jpg

-Matt