I guess its time to make this public and help out some fellow shield clickers.
What does it do? :
About 5 sec before searing flames the addon places a semi transparent frame above your UI which blocks all mouse clicks inside the semi transparent area. This will allow you to spam click over the shield till searing flames is actually cast. In that exact moment Atramedes starts casting searing flames the frame will be hidden and your clicks will register again. This will result in a very fast interrupt on Searing Flames (you still get 1 tick).
If you get more than one tick, you are not clicking fast enough or you stand too far away from the shield you are trying to click :P
What I personally do is have a G15 keyboard where one G key acts as a mouse click, then I set that key to repeat its function while pressed every 25 ms.
To show/hide the frame on will: /asc
To set the frames scale: /asc scale <number less than 1 default is 0.5>
Is this legal? :
Well, hard to say. In principal the addon does what any other bossmod do, it makes the encounter easier, but in my opinion this is really on the edge of what should be allowed and what shouldn't. I've been using this since day one of Atramedes progress, but the encounter is so bugged and has been tuned up and down so often it is hard to tell if Blizzard will ever decide to disable this addon or not. I'd say use it at your own risk (Altough I doubt you'd get banned for using it).