"Jack's Cup Runneth Over" is an orchestral piece from the Season 6 soundtrack.
Jacob sets down his torch and asks Jack for a cup. Jack provides one and Jacob fills it with water. Jack drinks it, and Jacob says he must hold the job for as long as he can. When the ritual is complete, Jack is like him.
Most of the piece is a variation on the protector theme.
The phrase "cup runneth over" derives from Psalm 23:5 in the Bible. (Note the occurence of the number 23, and the use of Psalm 23 during the Season 2 episode of that title.) It describes someone who is richly blessed. During the scene, Jack drinks from a cup.