An expander increases dynamic range, and like a compressor can work in both downwards and upward ways. An upward expander increases gain above a threshold, ie, the opposite of a standard compressor. However, in practice, we typically encounter them as downwards expanders, and in this format, any signal falling below the threshold is reduced in level.
The amount of gain reduction is again based on the ratio, and this is typically very gentle – 2:1 or 3:1, maybe. You’ll find other typical dynamics controls such as attack and release, but also some that are less common elsewhere such as hold, which sets a minimum period to sustain the gain reduction before the release can begin, and a range or limit (not to be confused with limiters and limiting) control, which affects the overall maximum gain attenuation applied – useful when trying to make the expander sound as natural as possible.