Behringer modelled this de-Esser after the dbx 902.

A de-Esser reduces vocal sibilance.

Press the "SOLO" button to hear only the filtered detector signal used to determine if gain reduction should be applied. Adjust the frequency, so it captures the sibilant sounds best.

Engage the "HF ONLY" button if you only want to affect the high frequencies of the source signal. The original dbx manual advises you to enable this for "de-edging" and "de-clicking" of "some instrumental material." According to the manual, for vocals, you should turn it off.

If you want to attenuate the signal more, turn the "RANGE" knob to the right. For vocals, it should stay in the "NORM" region, however. The manual advises us that values outside this range might sound unnatural when listened to for long. High attenuation might be helpful for instrumental material, though.

There's also a two-band de-Esser modeled after the SPL 9629 available in the FX Rack.

For more details on the original effect, see its owner's manual.

Screenshot of BDX 902 DEESSER effect