Is that true? That would mean almost 20ms^-2 AVERAGE accelaration i.e. -2g
Newton is turning in his grave...
That's simple physics a = dv/dt.
(hope moderators agree to kill this myth of large negative gees)
The graph of vertical speed is almost a straight line, therefore the acceleration "a" was almost constant for that segment. What that means? The PF didn't move the column control or the aircraft control system didn't respond.
I would put my money on possibility that PF passed out. The PM had very little time (10-12 sec) to notice and react, possible he was watching the middle console running checklist or programing FMS.