The App helps us calculate the length of circular bands when you need to change/replace them or simply shorten them to have them at a specific/desired stretch factor.
But it also allows you to calculate the length of the ropes/strings of a Mach Control system, the brilliant idea/invention of Stefano Soriano.
All info on the Mach Control system can be found on his personal website: https://www.stefano-soriano.it/index.php?option=com_content&view=article&id=31&Itemid=160