There is a class of gutter guard systems that work on a principle called water adhesion. Adhesion is the tendency of molecules that are not the same to want to stick together. Water is subject to adhesive forces more so than many other liquids because of its unique chemical structure of hydrogen and oxygen. There are several different varieties of adhesion, but the one that is at play with the water in your gutter system is dispersive adhesion. Two materials, in this case water and the gutter guard, are held together by intermolecular forces.

What happens with the rain water running off your roof and over the gutter guards is that when the water reaches the edge of the gutter guard system, and starts to fall over the side, the water adheres to the gutter system a fraction longer than anything else, allowing for the water to fall into the small slits on the side of the gutter guard. The picture below demonstrates nicely.

If it were not for the adhesive properties of water, this type of gutter protection would not work. Therefore you would have to use screen or mesh gutter guards instead.