User blog comment:Kogath/Monster Hunter X Discussion/@comment-5716529-20150911015536/@comment-5167887-20150911202014

^ HH 2nd Art grants a special power-up mode where, for a limited amount of time, all the notes you will play will be special "double" notes, which you usually obtain by hitting the monsters instead of playing in the air. If you play a song formed of double notes, it will play the song and encore the last one you played at the same time.