How do Grin rail anchors work?

[saucerful]

I'm interested in building a custom battery rack (will post another thread on that in due time). Meantime I am researching ways of mounting things to rear rack tubing. Looking at the photos of Grin's rail anchor kit, I see two screws per anchor. The one that goes through the rail (blue below) and screws into the anchor is clear to me. But there is another screw (red) that seems to only go through the anchor that confuses me. What is its purpose?

 

[bananu7]

Looks like it acts as a limit screw to prevent excessive compression force on the tubing, while still allowing for full tightening of the main screw.

 

[lnanek]

When they mount it as a pannier rack it's more clear that it doesn't stick out through the notch, at least:

You can see it doesn't line up in that case, so must be under the the rail.

 

[saucerful]

Looks like it acts as a limit screw to prevent excessive compression force on the tubing, while still allowing for full tightening of the main screw.

Ah super helpful. Thank you!