This beautiful manoeuvre allows the player to effectivley cross-strile (though without the normal striling requirements) between parallel lines with unequal vector potentials provided the L.V. of the originating line is greater than that of the receiving line and no Magenta stations are auked.
In 1888 MacLauren et al discovered that the Reverse Parry was also possible (from lower to higher L.V.) provided the diagonals had not been hatched within three moves, and all bridges were lowered as far as Kew.
For some reason Parry's Glide has fallen out of favour in the last fifty years, perhaps because Striles and Straddles are now considered standard play and are easier for the less able players to master; nevertheless the simple beauty of a well executed Glide (Parry's or otherwise) always adds a certain quality to a game and I personally would welcome their proliferation.