Had Peter changed the oil in is car, it wouldn't have exploded.
Had we jumped from that rock, we would have died.
Had Jim said something, he would have been killed.
This form of have is an auxiliary verb, it is used to make
Perfect tenses, and also to imply a subjunctive mood.
A subjunctive mood is when there is doubt in the phrase. All of the above phrases talk about events that
have not happened. Nor do we know if they ever will.
The phrases are hypothetical to say the least.
You'll notice that every time the auxiliary form of
to have is used, it is followed or preceded by
Had I eaten that I would have been poisoned.
I would have been poisoned had I eaten that.
Each sentence means exactly the same thing, literally. Other than the word order, an English speaker will interpret both sentence to have no contextual difference.