It's the caster angle that make the difference. I'm not an expert and this is how it's been explained to me from Blue Ox and others.
The caster works just like it does on a vehicle. Steering is very unstable with negative or neutral caster at the wheels. The steering wheel will not self center and after turning the corner you'd have to turn the steering wheel back to go in a straight line. At high speeds the wheels will wobble and make the vehicle unstable. So cars are built with positive caster. This forward projection causes the wheels to self center and become more stable at speed since they now want to maintain a straight line.
From what I understand the angle that's built into the hitch head creates the caster effect. The caster angle points rearward towards the trailer. This creates positive caster for the trailer. The bars and the caster work together to create a self centering force (sway control). It's what causes the trailer to want to maintain a straight line. Other trunnion hitches have less to no caster in the hitch head, so they have to rely on something else for self centering. Like the cams in the Reese Straight Line or a friction bar in others. The Blue Ox does the same thing without the additional parts.
Some say that you can fiddle with other hitch heads to create the same caster effect by adjusting head tilt all the way down. The Blue Ox has it all built in. Take it out of the box, put it on, and you're done.