The light gets brighter as the signal gets louder, causing the photocell to change resistance and, in essence turn a pot to reduce the gain. There are two major gain elements in use today. Altec also made one, and today, Manley Labs builds one using the same principle of gain control, but with a different tubes and making use of a full differential signal path which cancels out second harmonic distortion caused by the tube's action. VCA compressors are most common today due to the availability of expensive and reasonably good VCAs in IC form. Since this style of VCA is all on one chip, the designer's challenge is to keep the signal that's controlling the gain out of the actual audio signal path.
New designs are pretty successful, and offer the most positive control of gain vs. control voltage.
Just about everything inside the box affects the sound of a compressor - the gain control element, the way the side chain signal is derived and processed, the sound of the amplifiers in the input and output, and even the power supply. Of the much emulated classic compressors, only the Teletronix LA-2 had tube amplifiers. Certain compressors have a reputation for creating irregularities in frequency response, both favorable and unfavorable, but this in general isn't a result of gain reduction, but rather, input and output stages. The envelope holds a lot of the personality of a dynamic processor and can be used to give impact and punch. Depending on the envelope and the attack of a dynamic controller can make a instrument sound spanked or down right smashed.