rider is an AudioUnit plug-in that automatically adjusts the gain to keep the output signal at a fairly consistent RMS level. The gain is changed at a maximum rate of .5dB per second, so there should be no breathing sound as is common with compressors. The effect is useful for mastering, broadcast, or just plain listening.
The only parameter is Target RMS Level, which defaults to -14dB. That happens to be a quite common reference mastering level.
NOTE: The output is not limited. Lower the Target RMS Level, your master fader, or insert a limiter after this plug-in to prevent clipping.