Reppuzan (烈風斬, gale slash) is a technique that can bisect even a large-type Neuroi along with its core by letting magic power over-concentrate at the point of the blade. Both Sakamoto Mio and Miyafuji Yoshika require the sword Reppumaru to execute it.
Originally, this was something that comes under the telekinesis-lineage. Thus, for non-telekinesis-lineage Witch like Sakamoto, acquiring it was a next to impossible task. In part, she managed to do so by first mastering Kuroe Ayaka's "Unyou" and then using it as a foothold for her training.
A secret sword technique transmitted in Fuso since time immemorial, there is also an acme further above it: "Shin Reppuzan" (真・烈風斬, true - gale slash).
In Strike Witches Zero, it was said that Kitagou Fumika once split an ocean with her sword, just as Sakamoto did by using this technique. Considering that she was Sakamoto's master in swordsmanship, this might imply that the Reppuzan was originally a technique that belonged to Kitagou.