I'm not sure if this is a re post but I believe it isn't. I scanned through the 4 pages here and didn't find it so here it is...
Basically what you do is mash your binding for any spell, practice with your buff to get it right, and cancle it before the cast bar shows up. Now the little red text that says Interrupted should pop up. Try to fill your screen with as many as those as possible by mashing your ability and movement. Eventually your cast bar will appear and youre able to move around, loot or do anything while it casts. Once you get the hang of it you can pull it off pretty easily.
I've found this to work best with your mouse buttons. If you have a mouse with fancy side buttons with should be much eaasier then using your keyboard.
(This was working last time I checked; they may have patched this since I've last played but doubt it. Ill post pictures in an edit to comfirm!)