Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,170
It is the last stage of the process, when you are making payment. You must be logged in to your Iberia Plus account.
I would assume that you aren't getting a discount on the fare, but are getting a better return on your Avios when applying them against the fare.