Background on Senbonzakura
Byakuya Kuchiki's zanpakuto is Senbonzakura (A Thousand Cherry Blossoms). Its shikai is triggered by the command "scatter".
In its shikai, Senbonzakura's blade separates into thousands of slender tiny blades, which then fly away from the hilt. The release can be negated before it completes, such as when Yoruichi wrapped the blade in cloth during Byakuya's short battle with Ichigo. While the blades are too small to see normally, they reflect light in such a way as to appear as cherry blossom petals. Byakuya can control the blades at will, allowing him to shred opponents at a distance and break through almost any defense. While Byakuya can control the blades with his mind alone, using his hands allows him to do so more effectively, making the blades move almost twice as fast. Byakuya's Banka, named Senbonzakura Kageyoshi is essentially a much larger version of Senbonzakura's shikai. To activate it, Byakuya drops his sword. The sword passes through the ground and two rows of giant blades rise up from the ground behind Byakuya or surrounding the enemy. These then scatter into millions of tiny blades. The number of blades is great enough that Byakuya can use them for defense and offense at the same time. He commonly forms them into large masses of blades to crush opponents. Like with his shikai, its attack is triggered with the phrase "scatter".
Byakuya's bankai has several forms, achieved by arranging the blades in different patterns. They are activated by saying the name of the technique followed by the name of his bankai. Each form presents a different advantage, such as increases offense at the expense of defense. Byakuya's attack form is "scene of a massacre" or "annihilate", which unites the scattered petals into complete swords that form four rows around Byakuya and his opponent. In this form, Byakuya abandons the defense potential of his many blades and concentrates on killing the enemy. While Byakuya can control these swords as he does his regular blades, he usually attacks with just one in hand. He remarks in his battle with Ichigo that he normally only shows senkei to those he has sworn to "kill by his own hand". After immobilizing an opponent, he can call in the other blades to hit them while they're trapped. He can also use all the blades at once for one immensely powerful strike. To overwhelm the defense of an opponent, Byakuya can use "pivotal scene". This aligns every blade in a spherical formation around his opponent, which collapses into them from every possible angle, leaving no blind spots and no chance to escape. Finally, when Byakuya is weakened, he can use "final scene" or "last sight" This condenses every one of his blades into a single sword, drastically increasing its cutting power. The blade appears bright white, earning it the name "white imperial sword" or "white emperor sword". Byakuya also grows pure white wings and a halo-like circle in the back, both made of spiritual power.