Chromium intensifier (try 10% potassium bichromate + 10 % sulphuric acid for starters) work by intensifying weak silver deposits. If your film is properly exposed but underdeveloped, you'd be able to intensify the faint negative image to produce a printable one. Though lacking in density, an underdeveloped but correctly exposed negative has enough silver to represent the information needed to create a proper positive, but lacks density for proper printing. The chromium needs this metallic silver to attach itself to, and by doing so, increases the density of the weak negative image.
On the other hand, an underexposed, underdeveloped negative is a double whammy of sorts. Underexposure means that there is already less developable silver to begin with. Push processing only increases the densities of the exposed areas, but it will not create densities in areas which did not receive exposure. Getting the underexposed film underdeveloped means that the needed printing densities are not achieved. Less silver means less areas for the chromium to stick to. As it is, only the brightest highlights would be present- and intensification will only work on these. The shadow areas will remain blank.
Farmer's reducer and 2-part sepia toners can be used for intensification. Take only the bleach part of the reducer or the sepia toner. Bleach the low-density negative to convert the developed silver back to silver halide. Then use the toner bath of the sepia toner (stinking rotten egg-scented sulphide) to tone the image brown. Underdeveloped but correctly-exposed negatives lack printing density- converting it to brown will make it more actinically opaque to printing papers. Again, this sort of procedure works only with properly exposed or moderately underexposed negatives.
One old trick I read about (never tried) it is to photograph the underexposed negative using a modified slide copier. The weak positive seen when the negative is observed at an angle is the target. The slide copier has to be modified so that light can strike the negative's emulsion surface at an angle to
make the 'positive' visible.
The intensifier with the strongest effect I've used is the very toxic Mercury Bichloride
bath. But the resulting intensified negatives are quite unstable.
Jay