As a fellow guitar player I would recommend that you do not use this technique too often! In case you weren't clear on exactly what Patch was saying, the idea is to physically bend the wooden neck out away from the body of the guitar, thus slackening the strings and lowering the pitch of all the strings being played. While this will have a similar effect to the tremolo arm (whammy bar) on an electric guitar, it is likely to put your strings out of tune very quickly, will put unnecessary strain on the guitar's neck and body, and could affect the guitar's action and cause permanent damage to the neck, body, or the joint between the two. And as waterhouse reminded me, there is probably a metal rod, called the truss rod, running through the neck of your guitar, which is adjusted to raise the action of the guitar up or down by bending the neck in relation to the body. Bend the neck too much on your own, and you're likely to thow things out of whack in that respect.

The technique described works in reverse, too: If you pull back with your left hand, the strings will be pulled tighter, raising the pitch. But the same caveats apply. You might be better off trying a pitch bend the normal way: By pushing the fingers fretting the strings you want to bend up towards your nose. Of course you can't bend open strings this way, and this kind of bend is usually only done on one string at a time, when playing lead or soloing, when you can use three or more fingers on one string to get the maximum effect. You can only bend the pitch up this way, but if you need to do a fall, pre-bend the note above where you want to land, and then do a careful release.

So if you're playing a guitar without a whammy bar and absolutely must have a whammy-like pitch bend on all your strings, you can try it, but don't expect any Van Halen-like dive bombs, and definitely don't try it on your favorite axe!