This is a ridiculously easy question, so I apologize for bugging you all with it, but google has been less than helpful. Consistently when I watch videos of rocket launches I hear operators talking about propellant utilization being active (usually with individual stages singled out). What does this mean? Google was inconclusive, but seemed to mention both power generation from the engines and just general efficiency measures, but if it was either of these then why aren't they active at launch and only after? Thanks!