Some use the 1:1 ratio when detailing outdoors (i.e car shows) in the sun to prevent streaking and rapid-flashing. Never had to use this dilution but Sal does recommend this if one must detail in the conditions mention above.
Z6 is quite concentrated so it can handle the dilution and still perform very well, mild cleaning (i.e. fingerprint, bird residua, etc) and lay down some sealant re-enforcers.
Economically, its a great way to stretch the number of uses per bottle. :xyxthumbs