This isn't new knowledge however ~ seasoned casters have known about this for awhile. Basically, there is a point on your casting bar where your spell will finish, but it is sooner than the cast bar tells you and sooner than you are allowed to begin casting the next spell as a result. It is, so far as I can tell, attributable to the lagtime it takes for the castbar to appear and is applicable to essentially all "cast time" situations. Short cast time spells like Scorch or Flash Heal are particularily prone to falling behind over the course of being spammed, as the spell fires so fast that latency sometimes makes it hard for the castbar to keep caught up. I mean, spells are laggy by their very nature. I find knowledge of how this works useful at times as mundane as mounting, where you can start running at mounted speed before your mount is even completely summoned. It's a beautiful thing.
While this knowledge has always had limited application outside of situations that require moving around (like PVP, where you can instinctively begin moving around seemingly before your spell is done casting without cancelling the spell ~ mastery of which is part of what can seperate outstanding from average PVPers - see Vurtne), points have been made of actually using /stopcasting macros to benefit while stationary as well. The Quartz Casting Bar allows one to see exactly how much wasted cast time the cast bar is committing them to, based upon their latency at the time of the cast, as seen in the image below. The section in red at the end of the cast bar is essentially a buffer of lag representing the point at which, the spell will have already completed casting.
Now is /stopcast macroing hax? Not really sure. All it really does is in effect state "No, I'm not going to spend 1.7 seconds to cast a 1.5 second spell." It requires greater concentration to benefit from (since you cannot simply spam your cast button), is prone to mis-clicks that result in cancelled spell casts and even then, players with low latencies will not see much benefit. A poster on the Subcreation forums stated "I don't expect this to get banned, since it would make the game nearly unplayable for Australian players etc." which while a bit of an extreme argument, I think ~ is an interesting highlight of how the performance of our computers and quality of our connections can impact our performance in the game.