Chess Online

Chess Fundamentals!! 😀

First move absolutely must contain Black King.

Since second move can be a promotion, the B & N can form a barrier, confining BK to the 8th rank, so long as the N is on the same color square as the B (which is dark squares). Of the 7 possible dark squares this N can move to, which is the only one that will do this?

Answer: d5.

Notice the barrier, formed by the N & B:
b8, b7, c7, d7, e7, f8, and of course, f7 by White's King. Most importantly, the Kt on a dark square guards 2 light squares: b7 & d7. Knights always guard squares of color opposite to the color they occupy, because they change square color with EACH move.

(Actually, f8 is covered by the g7P, too, just long enough for it to Queen, but that doesn't really matter in this case. It could be important in other cases.)

Finally, promotion can be to a Rook (or a Queen).