At the moment the transistors are put on the surface of the silicon and then data connections are built on top and finally power connections. Backside power moves the power connections to the bottom of the silicon. This means there’s more space for the power connections so they will be bigger so have less resistance. This means the power will be more stable so thge chip will be able to run at higher speeds. There’ll also be more space to lay out the data connections so they will be better optimised / shorter so again more speed.
Also it will allow intel to charge more so thats nice…