Zoom B3n filter query

[NHM]

Hi 

I've been exploring the High Pass and Low Pass filters on the B3n.

The first knob on each filter determines whether the filter is set to HPF or Shelf. I get how the HPF works, but I can't figure out the Shelf - this, working in conjunction with the Gain,  seems to boost or cut all frequencies, not those determined by the Frequency knob.

Any insight on how the Shelf/Gain works, and how it differs from the H/LPF, would be greatly appreciated.