How do 3-phase dual voltage motor windings work?
I would guess that wiring for high voltage the motor would use all the windings ...


Correct.


  ... and when wiring for low voltage it would only utilize half the windings. 


Incorrect - although you could if you only wanted half power. The answer is to connect the half-windings in parallel. Note that in your second diagram T4, T5 and T8 are connected together forming a second 'Y' or star arrangement.

Then the outer terminal of each 'Y' is connected together. The Low Voltage 'YY' diagram shows T1 and T7 connected, etc.


  Where are the poles?


The poles don't enter into the problem of voltage selection.