I had always thought that a backlash had something to do with whips. Specifically, I thought that it was the reverse stroke, when the whipper's arm is being pulled up ready to strike, and there is a danger that the whip might strike an innocent bystander. Here is an illustration of my imaginings.
It fits the usual sense of the term perfectly: the violence that is being directed one way requires that it should suddenly be flicked back from the victim towards those who thought they were safe. Indeed, it looks here as though the chap in the white vest has turned his head to avoid the cat-o'-four-tails that the whipper is irresponsibly allowing to fly out into his blind spot. I can only assume that health and safety regulations were not as advanced in first century Judaea as they are now.
I was thinking about this because I read in the paper today that "the backlash over the golfer's sexual liaisons could eat him alive."
So I cast imagination to the sewer and decided to look it up. Backlash has nothing whatsoever to do with whips. It is, instead a property of cogwheels. No two connecting cogs can ever fit perfectly. If they did the friction would stop them moving at all. Moreover, in most systems a metal cog could heat up slightly and expand locking the device completely. As a result, you have to leave some room between the teeth and this space is called a backlash.
here. Video here.
Well, this stymied me. It means that the usual sense of backlash to mean a violent reaction is utterly wrong. The backlash is the empty period, it is the blank at the bottom of the page, it is the gap between songs, it is the please wait while your request is being processed.
But if a golfer were to get caught between the teeth of a very, very big gear system, I have to concede that he could be eaten alive.
Just so any whip enthusiasts don't get too disappointed, "enough room to swing a cat" is probably to do with a cat-o'-nine-tails.
P.S. Anyone interested in how much room is actually needed for feline swinging should check here. It's 10'1" for me, but obviously varies with height/size of mog.