User blog comment:Xilinoc/Ch. 571 (Yachiru's gonna wreck it)/@comment-11446176-20140305130913/@comment-1338810-20140306054607

A Zanpakuto can be in a technical released state and fight at that level without ostensibly changing form. Gin was able to activate and fight with Kamishini No Yari without having the blade glow or be massively long; he just dueled Ichigo with his sword in its ordinary small form, but at "Bankai level."

That's the way I've learned to look at Zaraki's constant-release Shikai. He doesn't know its name, so no sword transformation or extra abilities. But because of his reiatsu, it's constantly released, and therefore he doesn't fight with it at anything less than Shikai level.