Peak/RMS This contols how the signal level is computed, prior to comparing it to the set threshold level. At the leftmost position (full peak), absolute peak level is used, while the RMS mode uses an averaging technique similar to classic optical compressors. On certain program material the control will have very little effect, while on other (usually drum heavy) material, the effect will be more pronounced. I believe the ability to combine the two methods in any manner is unique to this compressor design.