Let’s say you have your basic rhythm section and vocals in balance, but you have heavy guitars and synth pads that are covering up the vocals in certain places. The problem is, that you like the level of the guitars for the most part and raising the vocals creates more problems. In the analog days, mixers used a technique called ducking with a compressor or a noise gate that had external control capabilities. The vocal track was fed into the sidechain of the compressor that was controlling the level of the guitar and synth tracks. This way, whenever the vocals came in, the guitar and synth pads would drop in level by two or three decibels, rising again when vocals were made their exit. Not only does this contribute to clarity in the mix, but also with a fast attack time and a short release time, a pumping effect would occur, which in a rock mix could contribute extra energy and excitement. The pumping phenomenon is also used on snare drums for the same reason. As with any effect, subtle application is required.