Forged Side Relief pistons (FSR) are made with a forging design that feature a reduced skirt width and a shorter wrist pin as compared to traditional style (sometime call full round forging.) The narrow skirt helps to minimize the contact the piston has with the cylinder wall.
If you have never watch any of the great videos uploaded by fiatnutz on YouTube, you should. Here is a link to the Costa Mesa R&D Automotive Machine Shop’s channel: