My real world experience -
30 years ago I was pulling my first TT, a 22 footer, with a short wheelbase Dodge van. I was going about 55 on a 2-lane highway when a semi went by me at a pretty good clip. Don't know if it was just him or a combination of things but the TT started wiggling - and got worse by the second.
I could feel the back of the van being pulled to the left, then to the right, back and forth by it's tail. If you have never experienced this, it is just plain scarey. It was a classic 'tail wagging the dog' scenario. You are wobbling back and forth and have no control. Luckily, I had the presence of mind to hold onto the steering wheel, keep steady pressure on the gas pedal, NOT TOUCH THE BRAKE PEDAL, and pull back on the trailer brake controller. This INSTANTLY stopped all the wobbling. I mean it stopped wagging around right now.
So, always know how to get your hand on that manual brake control quickly, without having to search around for it. I didn't have time to look for it as everything happened in just a few seconds.