The limiter (why it doesn't distort)

When you boost audio, its loudest peaks try to go past the maximum level a computer can play. Most boosters let those peaks get chopped off, which you hear as crackling. SoundBooster's limiter looks a few milliseconds ahead, sees each peak coming and lowers the volume smoothly just enough for it to fit — so the waveform keeps its shape and nothing is clipped.

It's on by default and does nothing at 100%. You can switch it off in Advanced controls; with it off, boosted audio is passed straight to your computer, which may crackle at high levels. More in why volume boosters distort.