All music is composed with condition that it will be listened through external speakers thus composer takes into account that some frequencies of Left and Right channels will be mixed in the air before arriving to the listener's ears. If listener is using headphones then no such mixing in the air happens and thus desired perception of the music composition is not achieved. Crossfeed technique simulates such mixing and thus listener is able to feel music as if it is heard from the external speakers, e.g. in the same way as music composer planned it to be heard.
Crossfeed DSP is the special technique which mixes some frequencies of Left and Right channels together. Crossfeed shall be used with headphones only and its goal is to simulate stereo perception of the external speakers.
Neutron provides several presets with different degree of mixing. Choose whatever preset you like or adjust effect parameters to your own perception.