The Community of Disc Golfers and About All Things Disc Golf
From Aerobie's website:
Tuning discs, by bending them, works on long range drivers. I do not recommend it for shorter range discs. A typical long range driver has a rim about 3/4" wide, which is about the width of a penny. Our new Epic driver has a variable-width rim which varies in width from one inch to 1.6 inches (fig 3 below). The wider the disc's rim, the more it responds to tuning.
To decrease the stability of any driver, you simply bend it downward. This can be done either by just flexing it like a taco, or by working your hands gradually around while bending just the rim. I prefer this latter method because it doesn't leave the disc looking like it just encountered a freight train. Fig 1 shows a disc rim being bent downward to decrease stability. Start at one place on the disc and repeat this bend about every 45 degrees around it until you are back where you started.
And of course, to increase the stability, you simply bend the rim upward (fig 2).
Downward bending reduces the height of the outer edge of the disc and that's what changes the flight. Upward bending increases the edge height. It's as simple as that.
Many players whom I've taught about tuning have worried that the disc won't hold its tune but I can assure you that your disc will retain its tune very well. I often measure the heights of disc edges with a special instrument that I made just for this purpose (fig 3 below). It's accurate to one half-thousandth of an inch and has proven that discs hold their tune indefinitely. This instrument has been useful to my disc research, but you don't need it to tune your own discs. Just go out to an open field and start bending and throwing.
Of course any disc, tuned or not, will become distorted if you leave it in a hot car with a pile of heavy stuff on it. So, if you're serious about accurate flights, you should store your discs properly without any pressure that might bend them.
Most players have noted that well-used discs tend to become less stable with time. It's often thought that dinged edges are the culprit. However I've deliberately dinged the edges of discs and observed no change in flight stability. Thus I think it's likely that the disc's edge has simply been bent downward from accumulated encounters with trees, rocks and other hard surfaces.