"Roger Long" wrote in message
.. .
[...] if the stagnation line has moved back under the leading edge, there
can actually be a small bit of flow with a forward motion relative to the
wing. This is very localized and only occurs very near the wing and only
at high angles of attack.

That's what I thought you said. And I think it's the same thing Tony wrote.

A stall warning tab could sense the stagnation line in one of two ways.
It can be blown up and forward by the reversed flow or it can simply be
spring loaded so that the switch is engaged when air flow drops below a
certain point as the stagnation line approaches. I'll confess that I
haven't looked at one closely enough to know which way they are set up.
They may even be different on different aircraft.

I haven't gone around inspecting the electrically operated stall warning
tabs, but all of the ones I've seen (on my own airplane, and on the larger
Cessnas) have been the former: a switch held down simply by gravity (and,
while the airplane is in motion, by the relative wind), blown upward as the
stagnation line moves below it. No springs involved.

Pete