WARNING: The following contains spoilers for Sasuke Shinden: The Teacher's Star Pupil by Jun Esaka and Masashi Kishimoto, available now in English from Viz Media.
In the Naruto series, both Naruto and Sasuke have a knack for remembering old fights and using key elements from them to evolve. It's something their master Kakashi taught them from a very young age, although it was more Naruto's style as he wasn't as gifted as Sasuke was with his Sharingan.
However, in the Sasuke Shinden: The Teacher's Star Pupil novel, which is set in the Boruto era, we discover that in his senior years, Sasuke has kept one lesson close to heart which he gained from a very deadly enemy.
This is a lesson learned from his and Naruto's first fight against Zabuza and Haku. Born in the Land of Water, Haku lost his parents at a young age and the bladed mercenary Zabuza would take the kid under his wing. When the Konoha trio fought them, while Kakashi had his hands full with Zabuza, a master of water and mist, Naruto and Sasuke struggled against Haku.
Haku used ice crystals to cut the boys up and were it not for Naruto's Nine-Tails Fox inside him, they'd have been slaughtered. He eventually overpowered Haku, but the kid retreated to take Kakashi's Lightning Cutter blade through the chest, shielding Zabuza from death. Zabuza would eventually die, redeeming himself and killing his employer, with the Konoha ninjas burying them next to each other as father and son. It was one of the most emotional arcs in the series, setting the tone for the franchise. Now, we know Haku's style is something Sasuke has adapted into his own arsenal after years of practice.
Haku's mom passed on the Ice Release technique to him, which enabled its user to manipulate water and wind in a chakra transformation technique. Clearly, Sasuke perfected his own version when the Naruto series ended. The five natures of chakra are Earth, Lightning, Fire, Water and Wind, so when Boruto sees Sasuke using it now in their training, he's confused as it's not his clan's signature one. But as Sasuke teaches him, the last Uchiha man can generate ice walls and spikes similar to Mortal Kombat's Sub-Zero because he studied Haku's manipulation of temperature.
An Ice Release user can lower their surroundings' temperature, generating snow if they wish, but Sasuke fakes it by using Water Release and rapidly cooling it with Wind Release, thus creating ice. He uses it to put out an explosion on a train in the novel's prologue, though when Konohamaru compares this process to Ice Release, Sasuke admits it's inferior. Still, he had to do it this way because using water to extinguish flames would have created steam and a flash explosion aboard the Thunder Train following this terrorist bombing.
In his continuing mentorship of Team 7, he specifies more about Haku's influence, indicating he and Naruto do regret Haku and Zabuza dying -- which would actually inspire Naruto to want to change the shinobi world. In addition, Sasuke actually stops using Wind Release and compresses water molecules to create ice to move a ship that has more bombs on it in the final act. It's faster and also, seeing as he can't risk any fuel from the boat spilling out into the sea, he uses the ice as a collector and springboard of sorts, with his Lightning and Lorentz Gun creating an electromagnetic field that allows them to shoot the frozen boat up into the sky for it to be blown up. In other words, he's adapting what he gleaned from Haku.
And it comes full-circle at the end when the mysterious Purple Moon cult use bio-electric pulses to co-opt Boruto's body, controlling it and making him a puppet. Sasuke generates pure water walls around Boruto, however, because they don't conduct the current, they break the line of electric transference. They're all throwbacks to Haku's manipulation of water and ice, leaving Boruto wondering what this sympathetic warrior was like in reality seeing as he played such a big role in how Sasuke evolved.
