Balances are an element of windows that is often taken for granted (until they malfunction). There are a wide variety of balancing methods currently in use but the most popular among the highest quality window manufacturers is the "Constant Force Balance" (CFB) system.
Other methods such as string and tackle, coil spring, tubular spiral and rope and pulley (traditional) are often fine for a time. However, eventually they all fail in some way or another. Some get out of adjustment, others have moving parts that can break or malfunction while still others have ropes or strings that can break.
We choose to use the CFB system because of their simplicity - they work well forever. The CFB has no moving parts! It consists of a 1/2" wide stainless steel band that is formed into a coil with a specific tension. One end is attached to the jamb and the other fitted into the sash. The sash is then permanently hung with a resistance precisely tuned to it's weight. The result being that the sash will stay where you put it. Whether you raise it 2" or 20" it will stay put.
The plastic housing in the picture above is mounted into a mortise in the edge of the sash. The "tongues" protruding from the housing are fastened into the jamb through the holes visible in the picture. Once that is accomplished the sash is then "hanging" by the calibrated coil springs.