The Procyon features a new nib design, with a hexagonal profile, which Platinum says gives it flex, for more responsive writing. There's only a small amount of line variation, but it does have a little springiness to it, which many people love.
The feed is a new design, too, with an inlet underneath the feed for the ink to enter when filling from a bottle, using the optional converter. You don't need to dip the whole nib and feed into the ink to fill it, as long as that hole reaches the ink. Platinum demonstrate it picking up the ink from a tilted bottle right down to the last drop. Less cleaning when you fill it up, and it makes your ink go further!