Propust is a compact tripple passive filter with with fixed cutoff frequencies for highpass and lowpass stages. It is a simple RC filter with 2 stages for HP and one stage for LP. There is a drop in signal loudness.
No power connection is required. Simple fixed filters can help you to clean up your final mix and make room for each sound, or they can be used for filtering noises and other signals when generating drum sounds, for instance. When the signal goes thru both HP and LP stages, it creates BP.