If you're talking about the plug between the two lines. If that plug is missing the oil circles back to the filter and so therefore it does not want to follow the main Galley and lubricate everything and build pressure the way that it
should. It just basically circles the pressure between the path of least resistance which in this case is back to the filter because the passage above it is more restricted it backs up makes a little pressure then the oil goes back to the filter back to the pump making a circle so in effect it is just rolling around in those passages.