I got lucky with this - as said, the link pops up in Manage My Booking, the same like it would offer a chargable upgrade. You can even bid for upgrades when you have a miles seat like I did...! Got a pretty good deal, converted the booking to a K and then earned full miles.

The only criticism I have is that:
a) You don't seem to get any email confirmations or anything from it. I didn't get a reissued ticket or an email to tell me that my bid had been sucessful - I just noticed it in Manage My Booking and then confirmed it in my credit card statement.
b) You have to put your payment card details right at the outset... so don't use a card that is going to expire/get maxed out/that you're going to cancel before your flight or they won't be able to claim payment if you 'win'...!