Pitting the plums is the time-consuming part of the procedure. Try to organize a party to pit the plums. (The people who brought the plums will probably not be players. If they liked to pit plums, they would not have brought them.)
Do not skin plums, as the skins are needed to color the sauce deep-purple.
Cut each plum with a knife and remove pit as best you can. Let some pulp remain on the pit, as it will be retrieved later.
Heat pits with clinging pulp in a sauce pan. In a few minutes the pulp will loosen. Place pits in a colander over a bowl and stir to squeeze pulp through.
To make sauce, 1 hour
Place all pulp, juice, and skin in a saucepan. From 2 pounds of plums you will have about 3 cups, nearly the same as the end quantity of sauce.
Bring to simmer. Taste and start adding sugar. How much depends entirely on how tart the plums are. If plums are tart, start with 1 cup of sugar. Taste as you go along and stir in more as desired. As this is a meat sauce, not a jam, a little tartness is desirable for many persons.
After 30 to 40 minutes, puree the mixture in the food processor. Return to heat and continue simmering.
Sauce is done when it is thick. Just how thick is a matter of individual preference. It should be at least thicker than a thick soup. Toward the end keep heat low and stir often to avoid burning sauce on the bottom of the pan.
When sauce is done, add garlic, ginger, and soy sauce. Suggested quantities are given, but are really a matter of taste.
Too much soy will darken the sauce, causing it to lose its purple hue.
If you are in a hurry, thicken with cornstarch or arrowroot. However, it is better to let sauce thicken by itself in cooking to concentrate flavor.
If you make this sauce, you have nearly made plum jam.
To make jam, omit the garlic, add a little more sugar, and cook somewhat longer.