One of my main interests of study of chess is underpromotion, the reasons why such an underpromotion is not only possible, but of necessity.
The most common underpromotion is that to a knight, which makes up over 90% of all such underpromotions (the other two are rook and bishop, in that order of popularity).
If player has to underpromote to a knight the most probable explanation is that he is trying to prevent a fork, check, skewer, or pin by his opponent.
This has happened in Master chess. But only rarely. And even rarer is when it happens more than once during a game.
Here is a delightful example.
USSR Ch., 1/2 Final
1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5 dxe4 5.Nxe4 Be7 6.Bxf6 gxf6 7.Nf3 b6 8.Bc4 Bb7 9.Qe2 c6 10.O-O (10.O-O-O is ECO’s suggestion.) 10…Nd7 11.a4 f5?! 12.Ng3 Kf8 (Obviously Black doesn’t want to castle kingside. But the text is not any better.)
13.Bxe6! fxe6 14.Qxe6 Nf6 15.Nxf5 Bc8 16.Qe5 Bxf5 17.Qxf5 Qd5 18.Qf4 (>18.Ne5!) 18…Rg8 19.Rae1 Rg4 20.Qh6+ Kg8 21.Rxe7 Qxf3 22.g3 Rg6 23.Qh3 Qg4 24.Qxg4 Rxg4 25.c3 Re4 26.Rxe4 Nxe4 27.Re1 Re8 28.f3 Nd6 29.Rxe8+ Nxe8 30.Kf2 Nd6 31.b3 b5 32.Ke3 bxa4 33.bxa4 Nc4+ 34.Ke4 Kf7 35.d5 c5 36.f4 a5 37.f5 Nb6 38.d6 Ke8 39.f6 Kf7?! (Black missing 39…Nxa4!) 40.Ke5 Nd7+ 41.Kd5 Kxf6 42.g4 c4 43.Kc6 Ke6 44.g5 Nf8 45.h4 Nd7 46.h5 Ne5+ 47.Kc7 Kf5 48.Kb6 Ke6! (48…Kxg5? 49.Kxa5 and Black’s king and knight separated and White’s pawns will rapidly advance.) 49.Kxa5 Kxd6 50.Kb6 Nd7+ 51.Kb5 Kc7 52.Kxc4 Ne5+ 53.Kd5 Nf3 54.g6 hxg6 55.hxg6?! (After 55.h6! Ng5, the kingside is locked up and White can concentrate on the queenside with moves like 56.c4.) 55…Nh4 56.g7 Nf5
57.g8=N (This knight underpromotion is to prevent a fork that follows after 57.g8=Q? Ne7+, winning the queen and White is also down a pawn.) 57…Kb6 58.Kc4 Ne3+ 59.Kb3 Nd5 60.c4 Nc7 61.Nf6 Ne6 62.Ne4 Nc7 63.Nf2 Ne6 64.Nd3 Nd4+ 65.Kc3 Ne2+ 66.Kb4 Nd4 67.c5+ Ka6 68.Kc4 Nf5 69.Kd5 Kb7 70.Nb4 (70.c6+? Kc7 with the idea of Ne7+, equalizing.) 70…Ne3+ 71.Kd4 Nf5+ 72.Kc4 Ne3+ 73.Kb5 Kc7 74.a5 Nf5 75.Nd5+ Kb7 76.c6+ Ka7 77.c7 Kb7 (The White knight is keeping the Black’s knight out of play.) 78.a6+ Ka7
79.c8=N+ (Another knight promotion for the same reason. 79.c8=Q? Nd6+, and White is going to find winning the game an extremely hard thing to do. It should also be mentioned that 79.Kc5!! also wins. But the second knight promotion is so beautiful!) 79…Kb8 80.Kb6 (White now threatens 81.a7! , winning the game with his last pawn.) 1-0