Could you be a more little specific?
A dummy ability, is an ability you make, this ability do nothing. You use it, and nothing happens. You can make one for example, using roar, eliminating buffs, casting times, etc, and animation if you want.
You use this, to launch a trigger (Unit - A unit Finishes casting an ability), then you compare it with the skill you want(so when the unit finishes casting the ability you want the trigger will start).
edit:woops, i started writing when there was no answer