Today is another technique I just tried - another cheap experiment with decent results. The first photo shows 'step one': I made my own ink pad simply by using a folded paper towel flattened in a disposable plastic flat-ish container. Moisten the paper towel with bleach (not so that it's sopping wet, just nicely moist). Rubber stamp onto a dark coloured card stock & let dry. You can see the results in the first picture. Make sure to wash the bleach off your stamp!!
Let the bleach dry on your paper. Next, you repeat the process, only using a permanent stamp in (I used black Staz-On). Don't worry if you don't get the stamp exactly in place over the bleached area, because you're looking for a 'shadow' effect.
The second picture shows three examples of what I ended up with. The one on the left was when I had too much bleach on my towel and ended up with very little detail. The middle one is on a darker coloured card stock and the one on the right is on a mid-tone cardstock. I think I like the right one the best. Have fun if you try this!