How do you explain why the A/S increases on thermal entry?

I had the delightful opportunity to fly with Derek Piggott last year
before the Senior Nationals and specifically asked him about this
observation. It seems to me that a thermal is a vertical gust that
would be noticed by the wing as increase in AOA and hence the airspeed
should decrease..but it doesn't? He said, (as best as I can recollect)
that "we used to call this the 'Yates Effect' but that this has pretty
much fallen from favor. A thermal with an accelerating core creates
somewhat of a venturi that will entrain surrounding air and will
manifest itself with horizontal gusts as well as the vertical as you
enter the thermal". If I took the trouble to graft it out, I would not
be able to illustrate this on paper as it would look like a tailwind
pushing me into the thermal rather than a frontal gust, but from the
cockpit of a G103 with the Jedi-Master it seemed a perfectly simple
explanation.

Gene