Solid metal and vinyl gutter covers work on the same principle; rain flows over the curved edge of the cover and into the gutter through a gap small enough to keep out the debris.
The solid PVC cover is excellent at shedding debris effectively. Made from rigid vinyl with a flex design that will fit most roof pitches.
The solid aluminum guard with no slits, slots, mesh, or openings of any kind. This style allows water to run down the roof, flow over the curve and into the gutter (surface tension), while debris slides off and falls to the ground.