Complex sentence: 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.
Split into simpler sentences: 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.