Its used like dragonrend - you hit a dragon with it and you dominate them. You can then mount up and ride piggyback as they engage in their existing flight and attack-run behavior against creatures/npcs hostile to you with some limited control like directing their choice of target. That part is a bit of a gimmick, but the useful part of it is if perhaps you feel like ending a fight quickly without the dragon having the chance to kill off nearby npcs - dominate it and just don't take it for a spin. I do imagine that if you do it too often you may find the slowly increasing number of dragons you didn't kill problematic at some point.