Twin is a passive balanced attenuator with two channels and six selection of signal reduction, that is: 0dB, -3dB, -6dB, -10dB, -15dB and -20dB.
At the back of the unit, for each channel, there are two 1/4" TRS jack connectors, one for the input and the other for the output signal.
Twin allows you to squeeze out the character of your preamplifier, in the purest form achievable, while still maintaining the necessary headroom needed during tracking.
Being able to exploit all of the sonic characteristics of a preamp is very useful, especially for the many professional microhone preamplifiers out there which do not have a volume control, as for example the Bad Dogs P1 and API™ 512c.
The same is true for those preamps that whilst having a volume control, also have an output transformer, as with the Twin you get the nice sonic saturation of the transformer as well, which it is not possible with just the volume control.