Not only will it keep you from progressing, but its presence is brutal to any audience. If you listen to a classical guitarist and you find yourself squirming in your chair, you can bet you're feeling the tension he's creating. We'll talk later about how to recognize tension and how to control it with relaxation - suffice to say that you'd be really surprised at how this ugly demon finds it's way into our playing. Once you've learned to identify tension, you'd also be surprised at how easy it is to eliminate. We've all been raised in an era of electronic music where the electric guitar is king and body contortions, twisted facial expressions, and all sorts of extraneous physical motions are almost required if one is to be considered a great player.
Classical guitar is a aural art form.
There is no motion of any part of the body except those parts which are directly involved in creating the music. There will be many references to this important principal in future lessons, but, for the time being think about your own personal experiences with this concept. The physical effort applied to anything should always be consistant with the task at hand. The required choreography of the fingers and hands rivals the most intricate ballet production. Don't underestimate the importance of these concepts - they are the essence of the physical requirements of playing classical guitar.