The 'stops' mentioned are wooden 'sliders' -- wooden sliding strips
located in the pipe toe-board, beneath the pipe feet.  The strips are
drilled with holes which, in the 'on' position, correspond with the
wind path from the chest up to the pipe(s).  Slid sideways to the 'off'
position the holes are moved.  Slid sideways to a stop pin the wind is
cut-off because there are no holes.  It's a simple on-off affair.