Essentially, what you do in this technique is take each word of your spec and exercise it. Ask yourself: What else could this be, and how would that affect things? Not all of these will be relevant, but it's a good way to stress each portion of the spec. Then simply chuck what you don't need.
For example: Mary had a little lamb.
- What if it's Bob? Does gender make a difference?
- What if there are 4 people named Mary? Who has the lamb?
- What if Mary is a nickname and her full name is something else? Does how you address it matter?
- What if she had it but doesn't any more?
- What if she doesn't have a lamb, but it should be here tomorrow?
- What if the lamb belongs to her but isn't in her possession? (i.e., what if "had" is not literal)
- What if there are no lambs?
- What if there is more than one lamb?
- How big is little? Is this age or size?
- What happens if the lamb is bigger than expected?
- What if it's a sheep?
- What if it's a chicken? (or some other non-related animal)
- What if it's a shovel? (or some other non-animal thing)
- What if the lamb is somehow unusual - color, or baa-ing?
This technique doesn't yield test cases directly, but it does help you ask intelligent questions about the system.
- This works particularly well when you're helping with requirements definition. The specs tend to be defined in a positive manner (what will the system do) rather than in a negative manner (what if the system doesn't)
- Exercising each word can help you break out of a thought rut that is causing you to miss some test parameters.
- It's a fast exercise to train on and can be used with non-testers fairly easily.
- This really only works if you have a fairly short spec. I can't imagine doing this with a 20 page document.
- Non-comprehensive. I don't know of any technique that is fully comprehensive!
- It can bring up overlapping test areas. Post-processing is necessary to distill this to testable items.
- It can be tedious if you have a long example. Schedule this one in chunks so people don't burn out.
Good luck! Oh, and don't worry about sounding funny walking around saying, "MARY had a little lamb. Mary HAD a little lamb. Mary had A little lamb." It really does help.