York millennium ycch163 rotary screw compressor

I understand the slide valve operation, but I do have a question about when the system is down. On this particular chiller there is no external oil pump to drive the slide back to minimal position for start-up. I can only assume (which I hate to do) that there is an internal spring that forces the slide back to minimal position, or ?? Thanks.

Yes, the spring is forcing unloading.
Josef.

What's the compressor? Is it the dxs?

If you're having load / unload issues, it's not unknown for the springs to break on these and require replacement.