"Red Seven" -- WHY Does It Work?

Can someone explain in simple terms "why" the Red Seven unbalanced counting method works?

I understand the basic reason why plus-minus works, with true count conversion from running count (i.e. dividing by remaining decks) but I can't seem to see clearly WHY the Red Seven method of counting +2 more for just the red sevens eliminates that TC need, and allows only running count (past the pivot point) to be accurate...