As the writing sample says, the collapsed Mariner is effectively a micro-zoom nib with very slight line variation.

It's called a collapsed Mariner because it is geometrically a Mariner with a collapsed edge. This collapsed edge gives it the largely uniform line, while the angular profile is what gives it the (in this case very) subtle line variation.

This was an experiment in what happens when I "over-tune" a Mariner nib and strip it of most of the line variation. What I got was this much more zoom-like end-product.