When you warm up, each of the body's energy systems is allowed to act as a «starter motor» for the next energy system down the line: ATP - PC acts as a starter motor for
anaerobic glycolysis (sugar breakdown), which does the same for aerobic
glycolysis, which does the same for the Krebs cycle fed first by
glycolysis and afterwards by lipolysis (fat breakdown), which eventually does the same for ketosis.