The mod helps you "dial in" your skimmer a lot more precisely... By opening and closing the gate valve, you raise and lower the foam head height...
The mod is simply 1 1/4" PVC that replaces your riser... Pull out the old riser, put in one of the shorter pieces, then elbow, then other short piece, gate valve, last short piece, T connector, then long piece out the bottom of the T... Hope that makes sense... I sold mine when I upgraded, or I'd show you a picture...
I recommend either buying the gatevalve mod, or making it yourself... It does help quite a bit IMO...