This is much more forcing than 5.O-O, which allows Black a number of options -- including transposition to the Max Lange Attack after 5.....Nc6.
The counter-attack on White's Bishop is practically forced here. Unlike the better-known Modern Variation of the Two Knights Defense (1.e4 e5 2.Nf3 Nc6 3.Bc4 Nc6 4.d4 exd4 5.e5), where Black can safely play 5....Ng4 as an alternative to 5....d5 due to the double-attack on White's e5-pawn, here Black has no satisfactory alternative. On 5....Ng4 White can play simply 6.h3 Nh6 (an ignoble retreat to the edge of the board) 7.Bg5! Be7 (forced as 7....f6? 8.exf6 hands White material) 8.Bxh6 gxh6 9.Qxd4 with a clear advantage in development, pawn placement, and King safety.
The best move. 6.Bb5+? Bd7! gives White no advantage. Black's next three moves are forced.
Best here may be 9....Be7, when 10.Bxe7 Kxe7! (10....Qxe7 11.Nxd4 +=) 11.Qe4! leads to complex play. The Queen move allows for a nice pseudo-sacrifice.
This move loses by force, as the remainder of the game demonstrates. Black must play 10....Bb4! leading to exciting play after the wild 11.O-O-O!? Notice that Black cannot move the Queen easily without allowing an attack by Nc3-e4-f6+, and if 10....Qf5? then the Knight attacks in another way: 11.g4! Qxg4 12.Nd5! winning material.
The Bishop and Rook coordinate in their attack on the back rank: if the Queen moves then 12.Rd8#. The motif should be familiar to all chess players from the famous game Morphy-Duke of Brunswick and Count Isuard played at the Paris Opera House in 1858. Black's Queening threats add a pretty turn to the scene.
No better is 11....Bxf7+ 12.Kf1! cxb2 13.Rxd5 b1=Q+ 14.Rd1 .
A desperate try, but there is nothing better. If 12....b1=Q 13.Rxd5 wins in all lines. Here at least White is challenged to find the only winning move.
Before making his move, my opponent felt compelled to seek out the tournament director and find another Queen to put on the board. I assured him that there was no need, since one of the ladies was coming off immediately, but I agreed nonetheless to stop the clock. Once the second Queen was secured and placed on the board, however, I was very glad for my opponent's careful following of the rules. The position was suddenly quite lovely to behold and I hesitated for a full five minutes, I think, before spoiling the scene.
The second Queen follows the first, selling herself as dearly as possible and encouraging a careless blunder: if now 15.Qxf1? Bxd5 White has no clear winning plan.
White's clear material advantage assures victory, but Black's exposed King allows for an immediate conclusion.
Black must lose more material or be mated in any case. On 18....Rxg5 follows 19.Qxf7+ Kd8 20.Qd7# and on 18....Nxe5 19.Qxe5+ Kf8 20.Bh6 pins and wins the Rook at g7. The chosen move allows for a rather pretty transformation of the scene: White's pieces now stand in regimental order along the fifth rank, and with a sudden charge they turn the flank to stand in a pretty line along the file to deliver mate.