Interestingly, the two Kosintseva sisters are both theoretical experts on this variation. They first encountered it three years ago, in their game at the World Cup Women's Rapid event, where Nadezhda made an extra winning attempt with 15.Kh1, but then agreed a draw after the reply 15...Be5+.
Eleven months later, they tested the variation again, at the Russian Womens' Championship. This time, Tatiana was White, and no doubt felt that confronting her fearsome opponent with her own favourite weapon would place her at a serious psychological disadvantage. However, after mobilising all her mental strength, Nadezhda also managed to draw after 15...Be5+. At Hangzhou eight months later, Nadezhda tried her hand as White again, having prepared the crucial improvement of 15 "I offer a draw". At the Rostov-on-Don Women's Grand Prix event three weeks later, Tatiana showed the extent of their sibling rivalry, by trumping Nadezhda's improvement with a TN of her own - she offered the draw after 13...Bh2+. And then, two months after that, she went one better again, taking the white side of the variation and unveiling the stunning further improvement, 13. Kh1, "I offer a draw".
It seems that yesterday's game was an acknowledgement that, under Sofia Rules, where a draw can only be agreed so early by threefold repetition, neither side can improve on the 15-move version of the game.
I trust readers will be as inspired as I am, to see two young professionals so pushing the boundaries of scientific exploration in our game. This is where we are so superior in chess, of course - in other sports, the spoilsport officials would have the two admirable sisters arrested for match-fixing!
Laughing all the way to the bank - the Kosintseva sisters, visibly racked with nerves, before their latest thrilling encounter (photo: Anastasia Karlovich/Chessbase)