Probably has to do with the tach circuitry. The kickdown switch momentarily shorts the ignition out, so no signal to the tach. Depending on the dampening, etc. I can’t say I’ve ever looked at the tach in that circumstance, and I’ve had electric tachs in ‘birds. Years ago.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Notice on my spring outing that when I put the gas pedal to the floor to engage the kickdown that the Tach drops to zero for a moment and then it seems to do the kick out and accelerates. Don't remember this working this way when I had one back to many years to remember. Do not know if this zero reading on the tach is a sign of issues?  Could some please explain ?  Thanks