Thanks for some help from David McGibbon, the master of all things Festool, for this answer.
The counter flange is the inner flange once the blade is removed.
It is keyed to the double flat of the arbor shaft and It normally just pulls straight off the shaft.
However, a build up of sawdust might be making it stick in place… or, the worst case might be that the blade torque has hammered the inside of the flange against the flats of the arbor. If this is the case, remove the blade cover and grasp the flange with a channel lock pliers and twist the flange back and forth as you pull.