This phrase comes from my dad, who is of Bristolian stock, so it may be highly regional. I've only heard it spoken, and not written down.
He uses it, I believe, when it looks like somebody is straining to accomplish something, and taking a more relaxed approach would be better for all involved. The exchange usually goes something like this:
Dad: You know what happened to the man who forced his pig, don't you?
blank faces and/or groans
I think the humour comes from the ambiguity over whether the man or the pig died, and what the pig was forced to do. That ambiguity also covers the fact that this strenuous activity is probably bad for the man AND the pig, no matter what it was.
Have I got the meaning right, and where does this actually come from? My dad doesn't know, but he's sure he didn't make it up. I've also seen the phrase/expression mentioned elsewhere but I've not found any answers as to its meaning and origin.