Knotted have you ever driven close enough to a semi truck on the freeway (under 2 car lengths or so) and you almost can feel yourself getting "sucked" in towards the vehicle? I think it is called slip streaming. (Note: don't try it if not as it is illegal.)

It works the same with the aerodynamics of cycling. That is why it can be dangerous if both riders aren't careful. It allows the draftee to coast.