Stereo Widening
Using some stereo widening processing is very useful on those mixes that may have been recorded in mono or just doesn't have a wide enough spread in the stereo panning field. It is kind of similar to the chorus effect but without any modulation or phasing of the original sound.
How much the stereo widening is applied depends on how narrow or mono the mix sounds an you don't want too much widening on songs that have a good pan spread of sounds. If a sound was panned a little to the left and another to the right, adding more stereo widening would put those sounds even further to the left or right.
The point to get to basically from experience is that when this effect is used a little bit, it can have a nice enhancing effect especially for headphone listening. Too much and you have another way of destroying a good mix that nobody will be happy with.
There's good & bad with all these enhancements thus far. But in the right hands of the experienced mastering engineer with well trained ears, they can be used to make a song sound it's glorious best.