A crank is basically like a regular noseband except instead of attaching it right to the buckle, you loop it through once and then buckle it.
I think you're thinking of a flash noseband: http://www.horsedata.biz/acatalog/Sa...shNoseband.jpg
This is a drop noseband. I don't know anyone who uses them personally but I think they are more popular in other countries: http://www.horses-in-sport-online-sa...p_noseband.jpg
I went to a clinic for work this weekend with a lady that is all about safety. She was doing a tack demo and I asked her what the purpose of the crank was. She said there really wasn't a difference in the regular cavesson and the crank. It just evens out the pressure under the jaw a bit more.