What is a pull pull system and how does it work?

Caz,

A pull pull system is where you have active wires on both sides of the control surface.
This means that when you hook up your control horn (say on rudder), you have horns on both sides and wires coming from both horns to the servo.
Makes for a nice positive slop free hookup.

Hope this helps

It did help,ty. What are the circumstances when you would use a pull pull as opposed to a conventional system? bigger plane,more precise control for aerobatics?

Pull-Pull has two purposes. The most obvious is the fact that it does not need to rely on pushing the control surface thereby eliminating the chance of a pushrod bending under a flight load. The second reason could be merely servo location to arrive at the desired CG without having to add ballast.

Caz,

I like to use pull pull where ever I can due to the slop free setup.
Routing of wires is also much easier than routing of pushrods and cables.

Thank you. Is there anyway to rig pull pull to aerilons with just one servo?