This is an excellent method for eliminating possible solutions. The method looks like X-wing, but has only three corners instead of four.
The Theory :
A, B and C stands for possible solutions in the coloured cells as indicated in the figure here on the left.
The three corners of the Y-wing are marked (AB), (BC) and (AC).
The cell (AB) id the key to the solutions. If the solution of this cell is (A) then, (C) will occur in the lower left corner. If the solution of the cell(AB) is (B), then the (C) will occur in the upper right cell.
Whatever happens in cell (AB), there can never be a (C) in the lower right corner. This means that the (C) can be eliminated from the lower right corner.