Very carefully! If you insist on doing this, and I don't recommend you do it, play an open string and push forward on the back of the headstock, you'll hear the pitch lower. Be forewarned, this can lead to broken necks, headstocks, and can loosen the neck/body joint on set neck guitars. If you've ever listened to Pat Travers Live album, you hear a bunch of neck bending all thru it.....