What the control stick is for the full scale pilot is the handle for the control line flyer. This gadget decides on lifes and death. As in all areas of modern life handles have developed from crude sticks to high technology instruments with multiple adjustments and heavy weight instruction manuals. I’ll try to explain why we dearly need this - or why not.
While there have always been simple tools like the American EZ-Just and it’s many copies, control line flyers always wanted to have an adjustable item. The most famous was the Jim Walker handle with adjustable line length. This was copied, too. Don’t remind me of the German copy made by Graupner. At that time my main occupation was to untwist the lines which had jumped from the spool. Today we renounce on line length adjustment and concentrate on more essential areas. Let’s see what we need.

1) Of course we need to be able to adjust the length of the lines relative to each other. Even if I try very hard I never get both of them at exactly the same length. The same is true for the leadouts of the airplane. Add to this that we all have more than one airplane , and you can see that adjustable line length helps a lot. We are not talking about dead serious top level pilots only here. I have a fixed handle-line-set for each of my two best airplanes always ready to use. But I have more than two airplanes, and I’d feel it somewhat snotty to have adjusted line sets down to the smallest model. That’s where the adjustable handle comes in.
2) It would be nice if the line length can be adjusted in flight. I’ve seen it several times that a friend has crashed his airplane in a contest. For one or another reason he was forced to change the lines shortly before the official flight. With those non-adjusted lines he tried to fly the schedule - no chance! In a similar situation I can quickly adjust the lines and save the flight. However I understand that this feature doesn’t mix very well with a few other ones which we highly prefer.
3) To vary the distance of the lines at the handle is a good feature. It’s no exception if an airplane comes out a little too sensitive or too sluggish around the pitch axis. It’s possible to cure this problem by adding some lead somewhere. However sometimes we just don’t want to do this - maybe the airplane is flying quite right in itself, only the handle needs to be adjusted. The smaller the line distance the smoother the reactions of the airplane ( look at those cute team race handles and what they do for the pilots ). In this case we’ll have to choose a somewhat more complicated handle design. Usually a metal bar in front of the fingers will provide the necessary equipment to solve the problem. Many flyers use standard dimensions for their control system, and as long as they stick to these they can expect that the handle will suit this arrangement. However as soon as someone wants to try a design experiment he might soon discover the adjustable line distance as a true saving.
4) There’s even more to this adjustable distance. In some cases it might be helpful to adjust the line(s) vertically - and separately, that is. Let’s take an example: the airplane reacts very well to our “down-elevator” command ( outside manoeuvres ), but is a little reluctant on “up-elevator” ( insides ). Now we set the UP-line higher ( maybe the DOWN-line, too ). What happens? The UP-line works on a longer moment arm now ( moment arm is vertical distance from wrist pivot point to line attachment point ). There is a sketch which can explain things much better. Since the moment arm is longer, the UP-line is moved a longer distance, too, but the DOWN-line is not ( it’s closer to the wrist pivot point )! Hence - we get more UP-elevator for the same wrist movement. I’ve been told that this really works, and indeed it sounds quite reasonable.
On the left we can see : the Graupner "Meister B" handle, a nightmare! Centre: the Praktigrip, two nightmares since it had two spools! The Thimpledrome with two separate spools, reeled off and up independantly.
5) Those highly sophisticated handles allow for even one more setting. Since there’s already the metal bar bolted to the handle body, this bar can easily be adjusted horizontally. This means: the line attachment points are farther off or closer to the wrist pivot point. At a first glance this allows to increase the horizontal moving distance of the lines ( longer moment arm ), thus increasing elevator deflection . HOWEVER - the pilot needs more power to move his wrist ( remember the longer moment arm ). Americans have invented the term “overhang”. It defines the horizontal distance between wrist pivot point and the line attachment point . Very often the centre of the handle is used for measurements to describe the handle shape. For better understanding of what’s going on this is not quite correct, the handle is not tilted within the hand, instead the wrist is tilted.
To put it simply: a longer overhang makes the airplane feel much smoother when in fact it isn’t. And if you want to calm it down use more overhang. The opposite is true vice versa. Anyway, we can get on by not being forced to change our airplane - we just cheat ourselves. No problem if we can please ourselves - and the judges! Anyway, with overhang we have an additional trim device. I personally don’t like overhang. Being not too strong a person I fly as little overhang as possible. I remember contests in strong wind where I just had to hang on to the airplane, hardly being able to move my wrist - and pray !
Another experience: on a contest in Italy my friend Peter Germann let me fly his new Bob Hunt designed “Saturn” ( he’s brave). With Peter’s handle ( big overhang ) I wasn’t able to do the pattern because I was afraid of crashing his airplane: it was EXTREMELY sluggish and unwilling to turn, and I felt it had no line tension. Now everybody knows that Bob’s design is one of the best , and I’ve heard exactly opposite ( positive ) flying experiences from other pilots. That’s what overhang can do.
  I’ve included some sketches to show what I’ve been talking about. To make things clearer I like to exaggerate. Exaggeration can make things easier to understand.
That’s why some sketches are not exactly “scale”, but can demonstrate things better. I prefer handles which are rather “deep” ( that’s in the direction of the lines ), very flat, and shaped to fit my palm as good as possible. Such handles can be held very tight without much effort, yet don’t slip or move in my hand. I want to feel comfortable with them when I'm flying those difficult manoeuvres.
The sketch on the left shows how to build a simple yet fully satisfying handle, even with adjustable line length, made from a piece of wood and some nuts and bolts. The sketch on the right describes a fully adjustable handle with such features as overhang, bias, line length, and line distance. It is commercially available from several sources.        
The handle on the left is one of those fully adjustable tools which the real expert cannot live without . On the right is my own handle. It's built for in flight adjustable line length. The distance between both lines is 100 mm. This is exactly tuned to my control system in the airplane.