The S-IVB also continued to thrust at a lower stage by venting gaseous hydrogen, to help keep propellants settled of their tanks and stop gaseous cavities from forming in propellant feed lines. This venting also preserved Safe and sound pressures as liquid hydrogen boiled off inside the gas tank. https://ultra-low-power-microcont43084.theobloggers.com/40173027/little-known-facts-about-apollo2