Blades HRB215SXA 6049967 Blade holder bolt tightening

I changed out the drive belt on this older HRB215SXA I think a K0 version. I can't get the blade holder bolt to tighten without the engine crank turning. I was hoping that holding the blades would help but crank isn't directly connected to the blade holder disk.

What's the trick to torque up that bolt?



Thanks