They say:

Designed with our friend Brian Hamilton from smallsound/bigsound for OBNE demo boards, we thought it a reasonable idea to offer every bedroom player a way to not annoy those around them.  Simply plug in the output of your pedalboard to the headphone amp, plug in headphones and blast your volume.  Play whatever you like, you're the only one who can hear.  Or a buddy can plug into the additional Headphone output.  Headphone output is mono out.

Two 1/4 Headphone Output jacks allow two users to simultaneously listen.

9VDC