An etchant does not know which way is down. It attacks every exposed surface equally, so as the floor of the etch sinks, the side walls are attacked too, and the resist is progressively undermined from beneath. That lateral loss is undercut.

Undercut sets a hard limit on detail. A line narrower than roughly twice the etch depth will be undermined from both sides before it reaches depth, and it simply falls off the plate. This is why depth and minimum feature size are related rather than independent choices.

Additive oil works by forming a protective film on freshly exposed side walls while leaving the floor of the etch in contact with the etchant. The result is a shoulder that slopes rather than a mushroom that overhangs, which is what lets a die both reach depth and hold its detail.

This is also why plate and chemistry are specified together. The additive has to suit the metal — zinc and magnesium behave differently in the bath — and a process assembled from three unrelated suppliers usually blames the plate for a chemistry problem.