Easy enough. This was actually quite similar to the way he possessed Ichigo, watching his battles in the reflections in the glass of the sideways cities. Waiting for the moment their eyes meet.

Zangetsu climbs into the air so he can be close to the reflective ceiling, staring his own reflection in the eyes. Me. That's me over there. I need to be over there.

And just like that, he is.

But his guard is far from down. In case it's some sort of trick, he has his hand on the hilt of his blade, hanging from his back as usual.