Originally Posted by Kvet01

I gave it some more thought and it works like this

At outposts (MAN, KRK, WAW): show your card at check-in for obtaining the upgrade

At sub-hubs (AMS, CDG, maybe MXP?) where passengers from a number of different destinations are transferring for flights to the PRG hub: show your card at the gate. You will recognize the subhub by the 4+ daily flights to PRG

In PRG: show your card at check-in or at the transfer desk for a waitlisting - at the gate it is too late.

Not that difficult really.