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

I think you are onto the right answer here.

Try thinking of it this way. with the glider sitting the ground in
flight attitude (for still air, best glide angle) the wing will be
angle down somewhat. If you put a fan directly under the wing blowing
straight up(the Thermal) it would deflect a portion of the air back
creating thrust.

In the air this only happens initually as the glider accerates upward
it is also generating addtional thrust from the thermal. For example a
glider descending at 2kts encounters a thermal going up a 6kts. before
hitting the thermal the air is going vertically past the glider at
2kts. Upon entering the thermal the vertical air is going past the
glider at 6kts. The glider will accelerate both upward and forward
until the glider is as a climb rate of 4kts and the vertical air going
past the glider is back to 2kts.

Brian
CFIIG/ASEL

Brian:

I'm not comfortable with that explanation. The glider goes "up" only
in relation to the earth, not in relation to the airmass. In fact, the
glider is steady in the airmass -- descending at 2 kts (more or less)
-- and we only seek out the rising airmass because it puts us higher
relative to the earth. Your explanation makes it sound as if the
glider is staying steady relative to the earth while the airmass
accelerates from 2 to 6 kts. (If this is so, it is only very
momentary.)

I think Terry and Burt have the explanation that is both
aerodynamically accurate and something a student can grasp relatively
quickly and easily.

Thanks to all. Fred