The Rock House teaching staff field all kinds of questions about instruments. One of the things a lot of new guitarist want to know about is the adjustable neck on the guitar and how to use it if needed. John McCarthy gives a a quick explaination below from the "Ask the Teacher" section available to members of The Rock House Method.
Most quality guitars have a metal rod inside the neck that’s called a “truss rod” that is used to adjust the neck to have less or more of a bow.
When string tension is placed on the guitar neck, the neck will naturally want to bow forward with the tension, truss rods help us control and adjust neck bow.
To put it simply, when strings are plucked they vibrate in somewhat of an elliptical pattern and because this pattern is wider near the center of the neck and tighter where the strings are anchored, relief (bow) gives the strings room to vibrate without contacting the frets and causing a buzz. Because necks, playing styles, truss rods and strings gauges vary, neck relief can also vary from one instrument to another.
Tightening a truss rod forces the neck back and normally reduces bow.
Loosening a truss rod decreases resistance and allows the strings tension to pull more relief into the neck.
Other types of nonstandard truss rods are:
Dual action truss rods can not only be tightened to counteract string tension and straighten the neck, but when turned counter clockwise they actually add upward bow. Basically, they are an added insurance policy. While creating more relief is rarely a problem you will encounter some necks that are dead flat or back bowed even with string tension and a loose truss rod. With dual action truss rods you can "force" a neck to bow forward and create relief.
Dual/Double truss rods. Most commonly found on 12 string or bass guitars, some necks incorporate 2 truss rods inlaid side by side in the neck. This is used primarily with high tension guitars.
We hope this helps answer some of your questions about an adjustable neck. For more info you can log into http://www.rockhousemethod.com/ and find answers to many questions you may have about musical instruments and playing instruments.