42 is not only the "Answer to the Ultimate Question of Life, The Universe, and Everything" but also the number of the decibels this little box can crank out... and that's a MASSIVE boost. (In fact, 42 db means the signal is amplified 126 times, that's probably the strongest boost on the market).
Although you can use the 42 boost as a normal linear clean booster, its main purpose is to boost distorting amps and overdrive / distortion pedals... and it does the job extremely well. You can easily push your crunchy rythm sound into high gain lead territory without getting all mud and excessive noise like when you try with another distortion pedal.
The low noise / high headroom design of the 42 along with the passive bass and treble controls ensures that all you get is a tight, focused gain boost.
Put the 42 in front of your amp or pedal, set the tone controls to full on and the boost at minimum, switch it on and... nothing happens (except maybe some added sparkle to your clean sound due to the high input impedance). Now start turning up the Gain control. Your basic tone doesn’t change but you get more and more distortion, sustain and compression. The 42 stays clean until the Gain control is around noon, then it starts adding some distortion and compression by its own.
Every amp or dirt pedal reacts in its own way when pushed (and some don't like it at all). If the sound gets muddy with more boost, turn down the Bass control and it clears up. If it starts sounding harsh, tame it with the Treble control.
Be very careful with the 42 in front of a clean amp, it can get insanely loud!!