Answer: His zanpakuto's name is Zangetsu and he found it out about it (or more like forced to) when Ichigo trains with Urahara. Urahara cuts his blade down to its hilt. He is then forced to find out its name to unseal its true form.

Question: What is the name of Soifon's zanpakuto and what is the command that she says to unseal it (shikai).