I think gravity is slightly stronger in areas where there is water, & maybe oil, Gold
etc. in large Quantities.
And if you properly do like they suggest & hold the stick Very loosely, When you cross that path,
the slight extra pull does it's job.
at least to me it makes more sense then hocus-pocus would.
though I'd like to know where the very first person
to grab a forked stick and walk with it in front of them
got the idea. It's certainly doubtful it was 100% by chance,
some kid or Geek was holding a Forked stick while walking
pretending to be holding a horses reins or something.
then even thought to dig when the stick dropped.