Basically, the regulation system, which is typical of most watches. involves a very minute shortening or lengthening of the balance spring. An index regulator moves one direction or another in order to very slightly lengthen or shorten the spring.
The Jones "arrow" or index uses an extra-long regulator and unlike most it is pointed in an opposite direction. By doing so, there is a greater arc, and therefore an ability to make even more very small changes.
The major problem with all regulation systems like this is that there's no easy way, except trial and error, to translate a movement to an exact timing difference. For example, moving the index 1/10th of a millimeter --itself not easy to do exactly-- doesn't easily translate to a precise change in seconds, such as say +1 second/day. There is a lot of fine timing involved. It is not like clicking an "off/on" or fast/slow" switch.
Second, the index can slightly move in use. Over time. it might loosen very slightly or be jarred. While that doesn't occur much or often, a slight move can translate into a timing difference.
The index regulation system is used in 98% of fine watches, with variances in design. Lately, for Calibre 51xxxx watches especially, IWC has switched to a "free-sprung" system. This system is more sophisticated and seldom gets out of whack, but it much more difficult to regulate initially. It basically involves no index or needle, but instead weights placed on the rim of the balance, to increase or decrease its inertia (which translates to slightly slower or faster, respectively). The weights are screwed in the rim and can be slightly screwed in or out. But, again, a slight twist of a screw doesn't translate to an exact timing difference. Patek and Rolex also use free-sprung balances.
As you can hopefully see, adjustment is not easy to accomplish within narrow and precise timing parameters, and to my thinking is as much an art as a science. You also can Google "Walt Odets" for articles on watch regulation.
