The pre exposure or "bump" exposure like PE says gets the film off the toe by exposing it to the point of making a very tiny dot in the unexposed area. Any further exposure creates bigger dots. Without this the shadows would be totally empty of dots. This was normally done with a safelight with a yellow filter in it hanging over the vacuum back with an enlarger timer running it. You calibrate this to your process by experimentally determining the smallest dot that will make a discernable dot on the finished image, in my case it was on the newsprint after the press printed it from the plate made with the halftone. How the image looks at those in between stages can be a bit odd to those not used to working with dot gain and slur.
On the other end of the greyscale the highlights should be left with a white dot on the neg just big enough to create the tiny specks in the hightlight areas, mainly controlled by the main exposure and fine tuned by development inspection.
Remember too that not all lithographic films are the same "infinite" contrast. Cheaper line films had lower contrast and would make a fuzzy edge dot that is hard to calibrate and make good positives from. There were films made especially for doing halftones with and worked best with the respective brand of developer it was made for but usually did quite well with whatever litho developer was on hand.
There were also a few films made that would make halftones without a screen. I never had my hands on any of that so I can't tell you much about them and I doubt you would be able to find any very easily that was still good.