Each recording source has its own optimum recording level. Moreover, the sound level may vary during the recording. This is especially painful if you use a scheduler and not at your computer while recording.
Automatic Gain Control (AGC) is a smart system which will boost quiet inputs and prevent overload from loud audio sources. i-Sound Recorder adjust gain on hardware level. The advantage of hardware gain are that, unlike a software gain, it increase dynamic range of the audio signal.
AGC allows you to choose three levels of sensitivity. Aggressive mode change the volume with a big step, which significantly speeds up the adjustment, but it is not well suited for music recording. In this case the best choice would be Smooth mode, which changes the volume without spikes.