Sandra-4 is a dynamics processor (in the form of a VST plugin) we like to refer to as an "angry compressor". It was designed specifically to help in the task of keeping vocal tracks around a desired level, and it is really good at it - some might even say it is a bit "obsessive" about its job. In other words, Sandra-4 is NOT a subtle processor.
For that reason, Sandra-4 implements parallel compression techniques. That way the user can choose how much of the processed and unprocessed sound he gets at the output of the plugin.