I think Contrast and Recovery. Add in the Intensive Tempo (6x200 in comehome pace of last 200 of your goal 400m) and the Extensive Tempo (3x3x300m jog recoveries) as well as emphasis on “lift” (Triple Extension/Step-Over and Down action) during Speed Endurance and Max Vel development sessions. It’s a package deal but there is a Balance between High Intensity and Low Intensity as well as an insistence by me on “Adequate” Recovery at every step of the program construct.
Sorry I have been off-line for ages, moving house, takjing leave from work to renovate etc etc.