It is often useful to have positive control of the clapper on a shaper
or planer in order to prevent marring the work (from the tool dragging)
on the back stroke, and to improve edge life of the tool.  It is often
possible to use harder or more wear resistant tools with a lifter that
would normally chip when dragging back with an uncontrolled clapper due
to impact from bouncing.  These photos show the design and installation
of an airlifter made from scrap in the parts bin.  The cylider is double
acting, which is not strictly necessary for flat planing.  I wanted the 
option to positively close the clapper when planing down angles on
auto feed.