Stopcasting macros are basically something I use to speed up my dps. Quartz is a cast bar addon that I use to also help boost dps. Basically what happens is quartz gives me a visual of my cast time. Its kind of hard to explain, but I will try my best.
When I cast a spell (i.e. SB), I get a cast bar like normal with one small addition. Quartz automatically builds in for latency and tells me when it is safe to cast my next spell. At the end of my cast bar will be a small red section, when the bar reaches that point it becomes safe to cast another spell. This is basically because my ui hasn't recognized that I have finished casting, while the server has. Stopcasting works on the same principle. If I just cast a SB and wait, I lose some time in my next cast because there would be a delay between my screen telling me a cast was successful and the server knowing that the spell was successful. Stopcasting stops the cast of the last spell and begins to cast the next spell.
For example, I use this macro for my SB casting:
#showtooltip Shadow Bolt
/use 13
/stopcasting
/use 14
/stopcasting
/cast Shadow Bolt
I'm no expert in macros, but this one works for me. When I use this macro, my first trinket will pop off giving me a dam boost (assuming I have the right trinket in place), if that trinket is unavailable it will try the next. The /stopcasting command is for things without any global cooldown (which is always assumed my macros). Using Quartz, once my cast bar reaches the red section, I click the macro again. That stops my last cast (which was already successful because of the latency monitor in quartz) and begins the next cast. Less time between casts means more DPS.
I tie my trinkets into my dots the same way:
#showtooltip Corruption
#show Corruption
/use 13
/stopcasting
/use 14
/stopcasting
/cast Corruption
I do this with all my dots (inefficient maybe, but I'm no expert in macros). Corruption, UA, and Curse of Agony/CoD are all tied the same way as my SB (Curse of Agony/CoD are also tied into amplify curse). Using the /stopcasting command and quartz has improved my dps, plus allowing the extra spell dam to apply to my dots is quite helpful.
That may or may not have made any sense...I hope it did. Here's the link for the quartz addon:
Quartz | World of Warcraft @ Curse.com
Read around some of the comments there, they can explain it better. Or just install it and try it out yourself. Once you see it you'll know what I'm talking about. Just make sure you don't click the macro until you've reached the red or the spell will just be overridden. In other words, this technique is not for those who just button mash SB.
I'm pretty happy with affliction for PvP and do fairly well. I get slaughtered in arena, but that may have more to do with my noobness there than anything else. I would never use destruction in PvP, much more of a raiding spec. Aff gives you lots of flexibility for both raids and PvP as well as demonology. But like I said before....demonology in a raid requires a lot of micromanagement (which I hate to do in raids, I'm more of the jump around and dot things while laughing hysterically to myself).
I usually don't check the forums this often, but I try to check in at least a few times a day. If you need clarification or if anyone else knows more efficient ways, please feel free to comment. Helpful criticism is always....um, helpful.
Plus, I'm heavy into Kara and ZA and any criticism would be helpful to my raid group.
Anyway, that got way too long for a message. Hope it helps.