I understand why they say it is the "reverse of a compressor" as it effects signal below a threshold as opposed to above it. However as it doesn't "increase" the signals dynamics, but instead "gate" it into oblivion. Consequently, given the usual application of a compressor, it think it is a VERY misleading description.

With an expander, you are effectively expanding "nothing" in place of "something" as opposed to expanding "something" into even more "something". This dose not increase the dynamic range of the signal.

A compressor on the other hand reduces the difference between to the max point and the minimum which is reducing the dynamic range of the signal.

I guess this is why scale degrees start at 1 and not 0 !