Possible Duplicate:
Sentences using: [something] + have + they
Inversion in “Only when the virus introduces its nucleic acid into a cell does disease occur”
In a sentence like this:
Only when a gun is reliable, accurate and durable, do I trust my life to it.
Why do we swap "I do" into "do I"? Would both options be correct? I can't help myself but start parsing the last sentence as a question when the word order is as is.