L/R vs M/S Compression
While a stereo compressor is controlled by the stereo channel with the higher momentary level and treats the left and right stereo channel equally, the M/S Compressor is controlled by the higher momentary level of the M or S channel and treats the M and S channels equally, unless you use special settings to alter this behavior. Without using these setting, m/s compression is controlled almost only by the M channel. Since the M channel is nothing else but the mono signal of the left and right stereo channels while the S channel contains all spatial information, the level of the M channel is usually 10 to 20 dB higher than the level of the S channel in a typical mix. This results in a 'center controlled' compression. For a typical pop music mix this means that kick-drum, snare, bass, and vocals control the compression, since these instruments/vocals are usually placed in the center and - at least as far as drums and bass are concerned - contribute the main part of the total level.
With L/R stereo signals, matching compression in the left and right channel is necessary; however, small differences in the tracking will not cause dramatic problems. With M/S stereo signals this is entirely different. A very small difference in the level relation between the M and S channels results in a different base width of the entire mix. A difference of 2 dB between left und right during the regulation results in a displacement of the center of approximately 15 % of the distance between the two speakers. 2 dB difference between M and S however, result in a difference of 20 % of the stereo base width. If the level of the S channel is 2 dB higher than the M channel level, the base width is extended to 120 % which already is more than the upper limit of base enhancements that don't cause a loss of mono compatibility and an off-uncentered bass range. Since such differences take place during the regulation, the base width is modulated by the compression. A guitar that is placed half to the right will move around the original position modulated by the kick drum or the bass guitar or whatever else controls the compressor. To avoid these effects, very precise alignment of the tracking over the entire useable gain reduction range is absolutely necessary. This is not only important for the level but for the frequency and the phase response as well as both also alter the spatial image of the mix significantly. Using two mono compressors without link will result in exactly this behavior. The base width is dynamically modulated, the placement of the instruments on the stereo base is not stable and weird things may coincidentally happen or not. Every once in a while a nice effect will come out of this but in most cases using M/S compression with no precise linking results in destroying the mix.