By trigger do you mean how to you get it, or how do you use it?Probably will never be seen or answered BUT how do I trigger Simon's Dominate Skill? I followed the guide but it will not trigger.
To get it on Simon's path in the OE, go through the fighting ring until the black haired elite challenges you. Nalili will pull Simon aside and say not to fight her yet. If you have enough built up sexual training in SImon's path, you can learn the Dominate skill in the training yard.
If you are asking how to trigger it in combat, it should just be one of Simon's skills he needs SP for. Though it only works on Female enemies.