I see no one filled this in, so in case anyone still wondering, you touch in/out as normal and then once a week TFL does some batch calculation and refunds you all the journeys. Also to note its Greenwich to Island Gardens as long as Cutty Sark is shut.
Also the talk at the time was the freebie would last “for at least a year”. That year is nearly up, and as a regular user I wonder what will happen next. And if it is for just a year, what a weird one-off thing to do. Not that Im complaining. But weird.