Board Thread:General Discussion/@comment-24815992-20140619090507/@comment-24579076-20140823032921

Zaraki's pre-Nozarashi was in a constant-Shikai state, his reiatsu forced the sword into Shikai but because they had little no to communication it only released "half" of it's full potential. This is somewhat similiar to Renji's Zabimaru not releasing it's true Bankai until the Sternritter Arc due to Renji being "not worthy". Since Zaraki now learned the name, he can go into "full Shikai" and that's how that axe was made to be :P