Consider the example they give (quoted from page 59):
These are certainly clear (despite being provided with clarifying notes!) but so is:The lawn mower that is broken is in the garage. (Tells which one).
The lawn mower, which is broken, is in the garage. (Adds a fact about the only lawn mower in question).
The lawn mower which is broken is in the garage.The key is in the commas, of course. However, the question is whether this example is bad English. Not at all! Consider the following somewhat inane conversation:
"The lawn mower is in the garage."Did you note the key point? It came in the question in the second line: we use which all the time to distinguish between options. It is a perfectly valid use of the word. Now look back at the quote from Strunk and White and note that they use which in exactly the same way to explicate the that sentence!!!
"Which lawn mower is in there?"
"The broken one."
"Aha, the lawn mower which is broken is in the garage."
Say NO to Which hunting.