A quote from that link - Mass (in kilograms, symbol M) is a measure of the racquet's "inertia," a word that means essentially its resistance to change. The change resisted by mass is change in linear (straight-line) velocity. SW is Inertia if I have a 300 gram racket what is the SW? If mass were inertia all 300 g rackets would have the same SW. When you add mass anywhere on a racket you increase the inertia of the racket. Inertia is used to measure SW and TW so when the mass changes TW and SW changes. Mass dampens vibrations the softer the material used to add mass the more it dampens vibrations. For Instance, silicone added in the handle dampens more than lead. The nodes in a racket are changed by mass. The sweet spot is a node and when you add mass you change the locations of nodes. Pretty easy to see mass changes balance. Balance effects TW and SW.