As with finger substition, the most difficult type of foot substitution is in cases where one foot replaces another foot which is holding a sustained note; during the replacement, there is a risk of sounding unwanted notes or releasing the held note.
The most difficult type of foot substitution is in cases where one foot replaces another foot. This is as with finger substition. This foot is holding a sustained note. There is a risk of sounding unwanted notes or releasing the held note. This is during the replacement.