I'm toward the end of the process of putting koni coilovers on the rear of my 914-4 vintage race car. Probably a stupid question but I've searched and watched some videos.
How do you get the top nut to bite and tighten down without just turning the shaft?

You have to have external top adjuster. Shock top must have blade to fit into adjuster

There is a proper shock absorber socket to hold the the shaft.
The usual approach is a electric or air impact.
Or hold with a vice grip.

Thanks all, the top of the koni has a small nut and the blade adjuster. I was worried about putting too much pressure on the small nut but once I got brave enough it worked.

You need a deep well socket with the ext adj using a impact. Securing the washer with the tang, channel lock, allows you use the open end.