Your question gets right at the heart of the matter why one wants to spend a few dollars on a quality machine.
Backlash (the amount of free turning on the knob before anything happens) can have many causes, on cheaper tuners they tend to accumulate, with the result that there is a great amount of slack and high friction.
I go to great length to bring backlash to a minimum, often I succeed that there is no backlash at all, yet the gears turn exceedingly smooth. (I refer to Chuck's posting)
This has to do with the overall quality of the machining of the gear and worm. Gears are rated according to AGMA, which stands for American Gear Manufacturers Association. A gear in the cheaper machines will have a rating of about AGMA 5 or 6, a Formula 1 Race car gear has a rating of AGMA 15, which is the pinnacle of gear manufacturing. Graf Tuners gears have a rating of AGMA 14, which is exceptionally high.
In addition, cheap tuners use a) cheap materials (soft steel for the worm and brass for the gear and b) only a helical gear. This gets very technical but a helical gear meets the worm only on a point. A point by definition cannot bear any load. What happens when one applies the string load the worm will work itself into the gear until a surface (the mesh) is created. This of course creates slack or back-lash, and not a smooth surface either.
Graf Tuners on the other hand use the very best materials, Stainless Steel for the worm and a specially formulated Gear Bronze for the gear. These together have superior low friction and excellent wear properties.
Furthermore, the gear is a "fully enveloping gear" which requires extremely accurate alignment between worm and gear but offers in return right from ther start a MESH surface, thus the gear does not run in and will remain virtually without wear for a long time.
If you would like a more in depth explanation of all this go to my web-site http://www.graftuners.com
and check out the TECH section.