An (upward) expander acts the opposite way of a compressor. It makes loud signals that pass over a threshold louder while keeping quiet signals the same. A compressor with a ratio of 1:3 will require 3dB of additional input signal to achieve a 1dB increase in the output signal. An expander with a 1:3 ratio will produce a 3dB louder output signal for every 1dB in increased input. Neither a compressor nor a gate will affect the signal if it stays below the threshold. For that, you need a gate.

Behringer's integrated WING Expander has similar parameters to the WING Compressor. Besides how the ratio works, the critical difference is that you should apply the makeup gain to attenuate the signal rather than amplify it. Since signals over the threshold will be louder than in the bypassed state, you should set the makeup gain to compensate for that increase in loudness.

You'll probably not need an expander in live audio, but it's there for you if you do. If you want to know more about expanders, please check out this article on (opens in a new tab).