I'd like to expand on Gus's & others explanation. Wings aren't exactly more efficient in lift when they are level. A wing will produce the same lift reguardles of orientation, assuming all else is equal. The diff, which is what I believ Gus was getting at, is the vertical portion of the lift can change. It is only the vertical portion of lift that fights gravity. With dihedral, as the low wing goes level, it produces more lift in the veritcal vector; this outweights the vertical lift vector on the high wing, hence the low wing lifts more than the high wing and the plane levels, note that gravity is a key part of the equation. Likewise, the high wing reduces it's lift in the vertical and increases its lift in a sidways vector, the sideways vector outweights any sideways lift from the low wing, hence the plane turns toward the low wing. Maybe MinnFlyer can produce a wizbang photo that explains this better than my ramblings.