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Grats on not understanding how latency works >.>
Get quartz and you'll see how much your "actual" casting bar is, meaning that depending on your latency, you can actually stop casting at x seconds before the end of your cast and it will still go off. There is no designated number and I sure hope you're not playing consistently at a ms where you can get .5 seconds off on a cast time. Blizz has since made it so there is a check server side or something like that so it doesnt work as well anymore.
To do this:
#showtooltip [your spell here]
/stopcasting
/cast [your spell here]
If you use the macro when your quartz cast bar reaches the darkened area, your original spell will go off and you will start casting another one. However, this is not spammable because since the server side check trying to cast a spell will start to incur the gcd even if you cancel it.
This is really only useful if you want to cast one spell (a heal for instance) and then move. You don't need to stand still for the entire cast bar. However you can get kicked and have the spell still go off while the school from which the spell was cast will be silenced. I believe this is due to someone interrupting a cast within the latency area.
Yes thats exactly how stopcasting used to work. I wouldnt advise using that anymore tho. With the spellqueue system added by blizzard, if you want to chain cast something you can eliminate most of latency by starting to spam your next spell a bit before the end of the current cast. System queues it as next spell and sends request for it together with ending of your previous cast.
As Cliche said you can use stopcastings in case of a single cast, but you can also do that by simply moving when your bar hits the red part of your castbar.