All this talk of masses,forces,accelerations,AOA changes etc is
irrelevent. Its simply a change in the apparent wind caused by the
introduction of a new vector (the thermal or sink).

Lets start with a simple example. The glider is just a point fixed in
free space. Introduce a horizontal wind of say X kmh. The glider's ASI
would register X kmh. Now move the airmass vertically (up or down -
doesnt matter) by Y kmh. The glider's ASI will show an *increase* in
speed equal to the vector addition of the X and Y components.

Now since a real glider actually flies down a slight hill this changes
the relative angles of the vectors. The thermal (or sink) is still
vertically oriented (for simplicity) but the glider's vector is tilted.
I never can remember how to set up the vector triangle so I wont try
and describe it here. But the end result is that lift causes a
proportionaly larger increase in ASI. Sink is interesting - for small
sink the ASI drops but for large sink the ASI increases. The anomaly is
dependent on the gradient of the hill.

Peter