Sites A and B both include content from the spying website S, which sets some cookies. Now S can correlate your visits to A and B because it's able to read its

Sites A and B both include content from the spying website S, which sets some cookies. Now S can correlate your visits to A and B because it's able to read its own cookies. This change makes it so that requests A -> S (requests from A to S) and B -> S are treated as A -> S1 and B -> S2 instead. Now S1 cannot read cookies from S2 and vice versa, even though they are the same site.