The Collaborative International Dictionary of English v.0.48:

Snifting \Snift"ing\,
   a. & n. from Snift.
   [1913 Webster]

   Snifting valve, a small valve opening into the atmosphere
      from the cylinder or condenser of a steam engine, to allow
      the escape of air when the piston makes a stroke; -- so
      called from the noise made by its action.
      [1913 Webster]