The goal of this game is to turn the black triangulation into the underlying blue one. Each diagonal can be in three different states: green (it can rotate), red (it cannot rotate), or black (it is undefined). At the beginning of the game there are an equal number of black and blue diagonals and no green or red diagonals. Click a black diagonal to make it green.
Any given diagonal will be bounded by a single quadrilateral. That quadrilateral has a second diagonal. A rotation of a diagonal switches to the other diagonal of the quadrilateral.
The crucial point is that each rotation of a diagonal also causes the adjacent diagonals to change from green to red and from red to green, while black diagonals stay black.
The game is over when each diagonal lines up with the underlying diagonals in blue.