This BUFFER / BOOST / LOOPER / LINE-SELECT in its simplicity hides a multitude of applications:
By adjusting the LEVEL control to the minimum will not have an increase in volume but your MINIAMP behaves as a BUFFER, that is an active circuit which transforms the signal that comes out of our guitar from high to low impedance, causing the sound to deteriorate during its Drive to your amplifier.
It is recommended for use as BUFFER to place it at the beginning of the effects chain.
By adjusting the LEVEL control has a linear increase of our output signal with consequent volume increase (+18 dB), useful to saturate the input of a tube amplifier, or to increase the volume during a guitar solo, but without any distortion or unwanted equalization.
BOOST + LOOPER function
Inserting in the flow SEND effects (Flow) and RETURN (Return) other effects such as a delay, the activation of your MINIAMP it will fire 'simultaneously to BOOST, allowing in practice to turn up the volume for a solo and enter delay all in a single operation.
The LOOP is located before the BOOST, to prevent you send in saturation input of external effects.
The LOOP is activated by inserting a jack in the output SEND otherwise be excluded automatically.
Function LINE SELECTOR
And 'possible to divert the signal present on the input of your MINIAMP between the two outputs Output (LED off) and Send (led on) to select for example two amplifiers or two chains of different effects.
In this case the Boost circuit is automatically cut off and the circuit is completely true bypass.
(In this configuration should not connect anything in the Return socket).