How exactly does it work?
What does it do to the sample?
I was thinking about it and I could only come to the conclusion that a part panned slightly left (or right) would be panned further by the effect, maybe all the way.
Am I right?
yes, you’re almost right. the expander just add difference between channels to each of them,
i.e. %100 of exapanding:
A = R - L ; differences between R and L
Lo = R + A
Ro = L - A
simple but effective for some stereo samples…
Thanks. 
not for some stereo samples only … overall very important for acoustic variations for all stereo sounds (VSTi’s too) and the whole sound of your song … with phase-variations you can produce more or less spatially sounds e.g. and so on …