Well, as far as I can see, he spawns various objects like cars, rocks, trees, barricades, etc on maps. For instance, take Dead Center - Map 2. Those streets outside the hotel are just wide open and can be difficult for infected to spawn in and attack, plus, survivors rushing is just bad.
By adding objects, not only do infected have more places to spawn, but it stops survivors from rushing too much and could give infected teams more chances to strike. To balance the additional spawn places, he also adds weapons and items on vehicles and objects spawns as well as around the map. He does pretty good to make most of the stuff feel fitted, but for a few, he seems to go beyond realism and aims for just pure fun and entertainment. It's impressive stuff.
He doesn't know that I am recommending him to you guys. I don't know if he has ever played on GCFTW servers, or knows that GCFTW exists. I mean, you guys do compete against MGFTW, right?