I just double-checked using the ZBT rate for a property where I regularly stay. For the dates chosen (4 nights), the Best Available Rate (unrestricted) is $139/night. When I use the ZBT rate code, the rate for the 1st and 3rd nights is $139/night and the rate for the 2nd and 4th nights is $69/night. Exactly as I expected and in agreement with the ZBT Terms for this property.
By the way, the ZBT Terms may differ for each property. For example, in the reservation above, arrival must be Thursday - Saturday. So please be sure and check the ZBT Terms before making the reservation.