Basically the first 'square' will be made up of triangle 0 and 1, the second is triangle 2 and 3 and so on.
So, if your pick returns an even number triangle then the other triangle that makes up the square will be the next triangle in the mesh 'ThisTri+1' but if the pick returns an odd number triangle then the other triangle making up the square will be the previous triangle 'ThisTri-1'
So, if your pick returns an even number triangle then the other triangle that makes up the square will be the next triangle in the mesh 'ThisTri+1' but if the pick returns an odd number triangle then the other triangle making up the square will be the previous triangle 'ThisTri-1'