On a less humourous level, it just may be that the front edge of that black thing (which I note he called a "grass collector", not a "grass catcher") drags slightly on the ground and scrapes up some of the cut grass that the mower throws out forward. It may work a bit, at least if the grass were fairly long when it was cut. Pity it's a roller mower though, or it would have a better chance of success.
