What is this trickery?

 Fred Litchfield has just passed on to me a curious tale of chess brilliance combined with chess ignorance. Playing an online game, he was being outplayed by a lower rated opponent (who shall remain anonymous for obvious reasons) and was soon down a piece. His king was then caught in a mating net and after 42.g4+ it looked like he had been checkmated. But that pesky en passant rule game to the rescue and Fred avoided instant death with 42. ... fxg3 ep 

It was at this point his opponent (who had completely outplayed a 2300) asked "what trickery had just been conjured up" (NB The server even added a button to the move to explain the en passant rule). No doubt shocked by such magic, the immediate recapture of the pawn by White allowed Fred to sacrifice both rooks to leave himself stalemated!

Anonymous (2204) - Litchfield, Fred (2307) [B30]
Online, 06.01.2021

1. e4 c5 2. Nc3 Nc6 3. Bb5 Nd4 4. Bc4 e6 5. Nf3 a6 6. d3 b5 7. Bb3 Nxb3 8. axb3 Bb7 9. b4 cxb4 10. Ne2 d5 11. e5 a5 12. Kf1 Ne7 13. Ned4 Nc6 14. Nxb5 Ba6 15. Nbd4 Qb6 16. Nxc6 Qxc6 17. Be3 Rc8 18. Rxa5 Be7 19. Nd4 Qb6 20. Qa1 Ra8 21. Qa4+ Kf8 22. Nb3 Qb7 23. Nc5 Bxc5 24. Bxc5+ Kg8 25. Qxb4 Qxb4 26. Bxb4 h6 27. Ke2 Kh7 28. Rha1 Rhb8 29. Bd6 Rxb2 30. Rxa6 Rxc2+ 31. Kf3 Rac8 32. R6a2 R2c3 33. Ke3 R8c4 34. Ra7 Kg6 35. Rb1 d4+ 36. Ke4 Rc6 37. Rbb7 f5+ 38. Kxd4 Rc2 39. Rxg7+ Kh5 40. f3 Rb6 41. Be7 f4 42. g4+ fxg3 43. hxg3 Rb4+ 44. Ke3 Re2+ 45. Kxe2 Re4+ 46. Kd2 Re2+ 47. Kc3 Rc2+ 48. Kb3 Rc3+ 49. Kb4 Rc4+ 50. Kb5 Rc5+ 51. Kb6 Rc6+ 52. Kb7 Rc7+ 53. Kb6 Rc6+ 54. Kxc6 1/2-1/2