The S-IVB also ongoing to thrust at a very low level by venting gaseous hydrogen, to help keep propellants settled in their tanks and stop gaseous cavities from forming in propellant feed lines. This venting also preserved safe pressures as liquid hydrogen boiled off while in the gasoline tank. https://ultra-low-power-soc64196.worldblogged.com/40022958/apollo2-for-dummies