I don't have a sump pump (and don't need one), but I'm fascinated by the
backup pumps that run off of municipal water pressure.

Presumably water has to flow to drive these pumps, so where does the water
flow *to*? Does it get pumped out with the sump water or does it go into
the home's sewage drain?

It's a clever idea, but it seems like getting rid of the water they use
would be a problem -- I would think that the heavy rains that might
accompany a storm that would knock out power might also make the municipal
sewage draining system ineffective for sustained flows, especially if they
were widely popular in a given area. Which also raises the question about
what happens when everyone on your water line is using the same thing -- is
there a chance of loss of pressure, or at least enough to keep the pump from
working right?

I'm not a physicist, but it also strikes me that you couldn't use the
water-driven pump to pump its own water out (along with the sump water) --
it seems like there's a perpetual motion machine there. (Cue the Simpsons
"In this house, we obey the laws of thermodynamics.")

Obviously these things work and manage to overcome some of my naive,
armchair limitations. But how?