the metal bit is the actual part that decreseases the vibrating string length, so by pushing down you are making the string come into contact with this and the string vibrates from there to the bridge. NOT from your finger to the bridge. So technically, you should be able to push anywhere within the space between the frets, but practically, I guess, the further back from the fret you apply pressure, the more likely you are to get a buzz sound because the string might be slightly higher than the fret....
Correct me if I'm wrong, thats just what I assume happens, when you think about it logically
