What are the advantages of the Burley hitch design? Burley uses a polymer hitch (the black plastic) which allows some degree of movement between the trailer and bike:



Most inexpensive trailers use a spring to provide that flexibility:



I've only used the cheap spring hitches which can easily absorb the twists involved when the bike falls over or the trailer flips (don't ask). I wonder what would have happened with a Burley. It seems the polymer would have broken.  Are the Burley hitches stronger than they seem and do they have advantages over the simpler spring design?
The spring hitches seem to be a cheap/simple way to allow the flexibility needed to both bend and twist when the tow bike corners, but they have an annoying side effect. In my experience when accelerating hard on a bike with a loaded trailer connected by a spring hitch you can feel the spring stretch with each hard pedal pulse then snap back in to place with a bump.

So Burley might have this new design to eliminate this bouncing effect. 

Chariot/Thule hitches don't use a spring, they use both a ball joint and a flex polymer arm which makes them feel a lot better.  Their ball joint rotates easily for normal cornering and leaning, and the stiff polymer doesn't normally flex or stretch noticeably but can bend in extreme situations as a kind of 'fuse' to save the aluminium arm from breaking and/or the bike frame.