Long, short story:
Am bad-to-average FPS gamer, looking to improve. Saw an OSU! gamer flinging its "shots" left and right and tried my best to mimick the experience in an FPS. And it worked significantly – all of my shots felt more "secure", even with my aiming being rusty beyond belief.
And the "trick" to do this is simply leaving your "aiming hand" to aim (ONLY) while leaving your "non-aiming hand" to shoot and everything else. That's it.
I never used it after wotlk as I quit wow after and when I came back in BfA i was fine with WASD, i just relied on shift and control layers more.
I never liked it fully because on my keyboards E is offset from SDF noticably more than W from ASD.