A "destructive parry" is similar to a "destructive beat" in that to execute one you make a much stronger, wider action that you normally need to in order to make a larger impact on the opponent's blade. When executed properly, you send your opponent's blade wide of your entire body, not just valid target (and if they're holding a french grip, you may send their weapon off to another strip.)
This is done as a tactic to get the opponent thinking about you taking their blade. It is an intimidation tactic that lets you set up for a finesse move.
(Hit the blade hard a few times, they start to worry about you hitting their blade and hold back some, so now you can either make a small beat parry that they react to by pushing back hard and you execute disengage riposte or you can hit them in prep as they are holding back so that you can't take their blade.)
This is generally done in foil, can sometimes be useful in epee and will get you hit 6 times to Sunday in sabre.
There is more on different types of beats at the training section: <a href="http://www.fencing.net/drills/" target="_blank">http://www.fencing.net/drills/</a>
HTH,
Craig
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