Well if you like to read and do a search on "porting" "modded" "modifications" "woods ported" you will get enough info to answer your question a hundred times over.
In simple term it is modification to the stock muffler, a bit of whittling on some internal parts that leads to better charge induction and raised compression that will make a stock saw cut 25% or so faster as well as making it able to be heard for about a mile further. The need for speed once indulged in, is nearly impossible to shake, so be warned before you enter!