Hey! I just found this simple trick out when I was testing.
All you have to do is use Dark Swarm all around in the little square. Then, move one of your air units (Mutalisk, Wraith, Arbiter) to attack the supply depot. You will get the sound of the attack, and it will do damage, but it will be invisible! Please, give me FEEDBACK!
ADDITION:
So, how is it everyone?
If you need help, I'll give you a replay...
Just ask...
This is one of the most useless things I've seen. Not since it's a dumb concept, but it's not pratical at all. You need to swarm at least 3 times before the attack is lost in the swarm. There is no need for cloaked units. All you did was shroud the attack, it's not invisble, it's consumed by all the dark swarms you're supposed to place. You can still see the attack coming from the air unit. Mayve if the swarms were pre-placed, but where would we ever need to use this?
Well, it's not supposed to be practical OR useful. Just something.
And BTW, the cloaked units were nothing. They were just there when I was testing.
I'm to lazy to remove them.
If you have a unit attack and it's in your range of vision, you will here it. If a unit gets attacked, but the unit attacking is not in your range of vision, you won't hear it. If the attacker unit is in your range of vision and is cloaked, you will still hear it. The same goes for some spells (yomato, disruption web). This could be used by having a unit cloaked by an Arbiter that's out of vision, and having a unit attacking or using a spell claoked by that Arbiter just in vision but cloaked by the Arbiter and attacking something out of vision. This isn't too practical though, and using what I described above would be very limited but I suppose useful if you didn't know how to use the sounds from the disc/star.dat/broo.dat and yopu wanted a clear sounding attack sound withotu seeing something attacking....
I have an idea, how about you use cloaked wraith to attack a constantly healing (with triggers) supply depot...
This isn't really a concept...