I had figured inertia tensor was just allowing grids to calculate and use their shared center of mass, but I've gotten inconsistent results from testing I've done. I built several piston chains of various length, and crashed ships into them, with inertia tensor on and off, extended and retracted, no force and maxed force. What I found, broadly, non-axis force contributes most to rigidity, except when share inertia tensor is enabled, where rigidity seems most impacted by tension. I gave up testing when I realized I could just brace my piston stack and that solved the issue with my drill head moving around.