What is meant by the phrase Riding the Vents on a submarine?

as near as one can figure from somewhat ambiguous ( possibly to confuse outsiders) my l946 Fleet submarine technical manual describes it as a (Surface condition) where the ballast tanks are partly flooded and trimmed down so the boat is almost submerged but not quite and the ballast vents are open. There are no diagrams to explain exactly how this works, but one surmises the general idea. the sub is heavily ballasted but still in surface trim, it is riding low in the water, as if it were heavily laden.For some odd reason this term has never made the scene in action-packed submarine films- unlike Crash Dives, running on the batteries, Green Board- an indication that the electrical connections are ( rigged for dive) all hatches and openings that are normally closed- sealed) and so on. Maybe it was not a very widely-used tactic.