An etching technique used to produce areas of tone on a plate: (This speaks to the general dilemma of making anything but a purely line-based drawing in etching; ink will only hold on the plate if there is something for it to sink into. As such, just putting a bare plate in the acid will create a depressed spot on the plate, but the ink will only cling to the edges, and the etched area as a whole will not retain the value.)
In order to create a soft-ground texture etching, one first soft-grounds the plate and then makes a texture in the ground by laying fabric over it and running the plate and cloth through the press. The weave of the fabric will pick up some of the ground in the pattern of the weave, and when etched in acid, the metal will be eaten away along the lines of the thread. The close, evenly spaced lines of the texture will hold ink much like the rings in an aquatint, and create the appearance of a general tone. One can limit the amount of the plate being etched by protecting the rest of the surface with stop-out varnish; fairly detailed images can be obtained by painting on stop-out with a tiny brush between multiple trips to the acid, creating successively darker areas with each additional bite.
Of course, soft-ground texture techniques are not limited to textures created with fabric. Any number of various items such as paper towels, feathers, flowers, etc, could be used. Different imprints will change the overall look of the piece; even different kinds of fabric with tighter or looser weaves to them will bring more or less light into tonal swatches.
One advantage of using soft-ground texture as opposed to other tonal methods is that it is very fast to apply and to etch: in general, light gray values will only take a few seconds in the acid, and even the deepest blacks will take less than an hour. However, etchings created in this manner tend to have a very flat, cut-out sort of look to them which may or may not be desirable for a given image. Burnishing along the borders of where the stop-out was applied will lessen the effect, but it will also create a clouded, fuzzy feel.