You can see how it works if you go to Dummy and use DS and not use Imp swarm, after DS buff expires it will display the icon because you can get new Imp Swarm before next DS.
So if you use it with DS everytime, you almost never see it. For trolls it will show up much more, if they line their berserking with DS+impswarm. If you see my math few posts ago in this thread, and you choose to use Imp Swarm with Breath of the Hydra trinket, then this will show up more frequently and you can use this to line up with DS more.
There is also bug to make the icon to display at pull for no reason, i'll have to fix it and i'll post new one soon.