The kick down isn't really just a kick down device. It varies the transmission pressure. More throttle, more pressure holding the the bands and clutches. When you add power you need enough pressure to keep the components clamped to keep them from slipping. You added power and it slipped. Pretty simple. Take it out, you'll find damage. I'd suggest a manual valve body in your application. That way there is no throttle pressure linkage or cable.
Doug