CAUTION: This can be missed. The main requirement for this is to have 3 Heart Points with Finnel. Given there are only three to play around with (a fourth is available if you do a 2:1 playthrough, but save that for a secondary run to get Tyria/Cocona's endings), this is a crucial point. The following points are when you divert Heart Points:
After you have passed the third heart point, the point of no return for this moment, you will continue with the story. Refer to the next spoiler for the little step by step deviation.