Place as many jamun fruits as you have in a heavy pan, and add water to cover. (Save a handful of fruits to serve, if you wish).
Bring to a boil, reduce heat, and allow the fruit to cook. Use a wooden spoon to mash the fruit from time to time.
When it seems that the fruit is mashing easily (about 15-20 minutes), turn off the heat and allow to cool.
Once the mix is cool enough to handle, strain the liquid out, using as large a strainer as you can find. Much of the juice will remain in the fruit pulp, so use a spoon to mash and press the fruit as much as possible to extract the pulpy juice. Stop when it seems like the fruit is getting dry and the jamun pits are appearing more and more in the left-over pulp.
Now measure out the liquid you have, into a pan that can be set on the stove. Add sugar in a 2 cups liquid-to 1 cup sugar proportion, along with the salt.
Bring to a gentle boil until the sugar is incorporated.
Taste at this point -- and adjust the sugar and salt if necessary. You want a mixture that is slightly on the sweeter side, and just a touch salty.
Continue boiling on medium heat for approximately another 10 minutes. Then turn off the heat, allow the liquid to cool, and bottle.
Store this concentrate refrigerated, for up to a week.