The head down technique is not necessary. Charlie always said “head in line with the spine”. I believe Asafa artificially tries to extend his acceleration by thinking that keeping his head down is the key. This negatively impacts his transition to max velocity.
Watch Ben’s old races. He kept his head in line. And still reached 12.1 m/s on slower surfaces with less stiff shoes. His splits prove his acceleration was fantastic, as well as his transition to max velocity.
When we worked with Tim Montgomery, he told us that Trevor told him to “look for ants” (i.e. Keep your head down). This mis-information screwed up Tim’s start and accel until it was corrected by Charlie.
Long story short: keeping your head tucked over can be problematic and is not the sole answer. Be mindful of all technical elements and how them work in combination with each other.
I’m currently working on a video project that looks at start mechanics and incorporates much of what Charlie taught me about start and acceleration mechanics.