The dual para eq is a compact, parametric 2-band-equalizer, which you can apply to a signal chain to lift or lower certain frequencies.
Parametric means not only lifting or lowering, but also adjusting separately position and width of frequency bands. So it does not only enable you to equalize frequency response errors (hence the name equalizer) and to suppress feedbacks or resonance peaks, but it may also be used as a flexible tone control for purposeful sound manipulations.
According to its application in a signal chain, the input stage of the dual para eq has been laid out unlike an instrument preamp (e.g. for pickups or microphones) as pure level adjustment stage. With the phase switch the phasing of the processed signal can be reversed if necessary. Finally the entire dual para eq device can be bypassed by pressing the bypass switch, allowing for a comfortable A/B comparison.