Originally Posted by Nicholas Wyss
Neutral Spine isn't really flat, it's lordotic (arched) through the lumbar spine, kyphotic (rounded) through the thoracic spine, and again arched in the cervical spine. If you try to hold your spine in a position that is actually flat, your lumbar spine would be vulnerable to a posterior disc bulge.
while you are correct, many people hyperextend when they are told to arch their lower back, and when told to keep their back straight they go to a neutral back. It is not anatomically correct as you point out the spine in neutral is curved, but for cuing purposes thinking flat back usually works well.
OP for your purposes, tighten your lower back exactly like you are, but then clamp down on the abs also and you will probably go to the correct lumbar curvature. Lower back tightness is good, but not in the absence of ab contraction also.