Reverse curve gutter systems allow small debris AND large debris to enter your gutter.
These systems can be very expensive, and are not effective at keeping debris out of your gutter. Common reverse curve systems on the market are LeafGuard, K-Guard, Gutter Shutter, Gutter Topper, etc.
Reverse curve systems all work the same way. A piece of metal with a bend on the end of it covers your gutter. The bend allows the water to flow back into the gutter. Surface tension "attracts" the water, causing it to defy gravity and seemingly shed debris.
These systems only shed some large debris, but the small pieces flow with the water directly into the gutter. Over time, the small debris builds up and causes a clog at the downspout. How do you clean it out when it's covered?
During moderate to heavy rain, water overshoots the reverse curve guards like a ramp.