If the Do not unsynchronise ID3 tags option is enabled the tags will not be unsynchronised. Unsynchronisation is required when there is Metadata which could be misinterpreted as the start of the actual audio data, extra bytes are inserted so no misinterpretation will happen. But if you are using an application that does not understand unsynchronisation then the Metadata can be misinterpreted instead, this problem is most likely to occur if you have added cover art, the end result will be an image that does not display properly in some applications. Enabling this option will not usually cause problems with the audio because most players will correctly identify the start of the audio even there is Metadata which could be misinterpreted as the start of the actual audio data.