In a "normal" tube feeder the air system is closed. Squeeze the bottle to push juice through the tube to get to the cart. When you stop squeezing, the juice is sucked back down the tube into the bottle. The vent opens the air system (1mm brass tube w/opening burnished to about .2mm). This allows the juice to sit up in the tubing and easily flow the short distance to the cart.
Here's the inside of The Puck
The brass fitting that screws into the cart has an opening where the tube is attached. By adjusting that opening larger or smaller, thick or thin juice moves at about the same rate. You can use one size opening and change how you draw on the cart, but I'd rather have the hardware make most of the adjustments.