Disc flight can really be divided into two stages: High speed and low speed. High speed first part of the flight when the disc is going the fastest. If you look at Innova's 4 number system, this is what they call the "turn." In reference to stability, most discs are either high speed stable (meaning 0 turn at high speeds when thrown flat and correctly) or understable (-1 or more, so they appear to flip up from a hyzer or flip/track over from the flat position). I think a few companies have some discs that are actually overstable in the beginning of their flight as well (whippit and demon are two that come to mind). The low speed portion of disc flight (called the fade by Innova) is normally overstable (fades hard to the left) or stable (no fade). I looked at both the flight charts for Innova and Gateway, and neither company claims to have a disc that is low speed understable. Unfortunately, Discraft's one number system doesn't really help you about both sections of your flight, so you'll have to try them out yourself. Keep in mind that these numbers are only a reference, and you'll have to experiment on your own to see how a disc will fly with your stroke/snap/strength.