I said that air moves forward across the wing which was clumsy wording
since it could easily be misinterpreted. Air near the wing surface
above the stagnation line moves from front to back so, 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.

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.

When I sail, (which seems to be more than I fly now) I have lengths of
yarn taped near the leading edge of the jib. If I get to too high an
AOA, the one on the "bottom" of the sail will start to point straight
up and even forward. Even though the sail is still pulling hard, the
stagnation line has moved well around to the windward side. There is
a slight drop in efficiency but no dramatic stall.

Pop quiz class:

Sails don't stall and suddenly lose their lift causing the sailboat's
heel to suddenly decrease. Can anybody tell us why? (Hint: Assuming
you had long enough landing gear to get to stall AOA while rolling
along the ground, you couldn't create the same kind of sudden loss of
lift that you experience in the air.)

--

Roger Long