the cabbage will float and not stay under the brine. the brine is what ferments and preserves the cabbage. without the weight, you end up with a bunch of moldy mush at the top that looks like the world's largest loogie..
been there and done it...
edit: if there are pics, i don't see them. work filters them or something, but the rock goes in the jar on top of the cabbage.
-------------------- Never kick a fresh turd on a hot day..