I apologize for my ignorance, but is this "smoke in the harness" actually true???

I have NEVER heard of this (keep in mind, I am 26, and I have never owned nor worked on an old British car).

If it is true, can someone give an overall definition on how it works, or if it is not true, where does the joke originate from (smoking wires, shorting wires, etc?)...my educated guess is on the latter of it not being true.

Thanks,
Alan