It might be worth comparing how Fellini moves work and feel on the Pavoni (closed boiler) and on the Caravel (open boiler). On the Caravel any air in the group head chamber has an unobstructed upwards path into the boiler and indeed out into the atmosphere. However "Fellini" pulls are pretty much a necessity on this design to achieve full preinfusion with sufficient resistance at the puck to achieve appropriate extraction pressure and a full (but nevertheless pretty much ristretto) shot. If the feedback from the lever on the Pavoni was the same with and without Fellini, but it was just shot volume that changed, I'd be more inclined to believe that an air pocket was at the heart of the phenomenon.