Simply put, this is a design for a porch that is basically a large conveyor belt. If somebody knocks on your door and you don't want to talk to them, simply flip a switch and the conveyor belt turns on, conveying that person away from your door.
If you want to have fun with the person, or if the
is just really persistant, you have variable speed settings available with the turn of a dial. If you're not too worried about lawsuits, you can install a raised ledge, like the bottom of a doorjamb, behind them to trip them up. A simple "watch your step behind you" sign on the door ought to solve the problem of lawsuits.
If you're really ambitious, have the conveyor run all the way out to the edge of your property.