Many times people will "pull" on the string to increase volume, there is a better way to do it.
By "pushing" the string towards the face of the guitar, you achieve more volume with a good sound. Good sound, you say? If you pull the string you will achieve more volume, but at the cost of the sound quality - it will be harsh, twangy, which is something classical guitarists try to avoid.
In terms of acoustics, the string is being displaced, and the level of displacement is related to the loudness level. A guitar string is very similar, if not identical, to a sound wave. Both have crests and troughs, both have nodes and antinodes. In my acoustic physics class my junior year of university, we used my guitar as an example of how a sound wave moves.
So what am I getting at here? The way the string is displaced changes the tone. If it is displaced towards the face of the guitar, the string once played travels opposite of the face of the guitar. If it is displaced by pulling back towards an adjacent string, when released it moves towards another string, which is what we want to avoid.
I am linking a video showing exactly how to push the string rather than pull.
"Just push it."