Larry's explanation is 'right on' albeit lengthy. He covers the subject accurately.
I would add 3 comments:
First, if you are going to err on the percentage of tongue weight (he said 10%) make it more. It won't hurt to have 15%, for example, but it could have disasterous results if you drop significantly below 10%, as he stated. :'(
Secondly, I added a friction sway bar to my 2619, more for piece of mind than necessity. However, the wind effect from big rigs became a non-issue after I added the sway bar.
Finally, if you should EVER have your trailer begin to sway on you, the first thing to do is lightly apply the TRAILER BRAKE manually to slow down the entire rig... not the tow vehicle brake. Normally, if the trailer is prone to sway on you, it will occur while you are going down a hill (as opposed to flat terrain), so downshift to keep your speed maintained to minimize using the tow vehicle's brakes and apply trailer brakes only if you detect any sway. Then, bring the entire rig to a safe stop off the side of the road and inspect all your wheel lugs, reset your load (if it shifted) so that 10-15% of the total trailer weight rests on the tongue (hitch). 8)