In a highway down-grade condition (reference video), "my" initial reaction would be to maintain straight steering while 'manually' applying the trailer brakes (@ brake controller), letting up on the gas pedal to reduce TV speed and once TT sway is under control gradually apply TV brakes.
I've never had to do this on the open road, but when I started a trip with my TT, I would apply the trailer brakes while driving through my neighborhood at 15 MPH.
It accomplished two things. One it verified my trailer brakes worked. Secondly, it helped to familiarize me with the location of the manual trailer brake switch. My hope was in an emergency, I could find the trailer brake switch and apply the brakes without taking my eyes off the road.