My only real idea for how Segunda Etapa might work is that by casting away the Heart, it- for lack of a better term- 'unclogs' the pathways between someone's spiritual power and the outside world, as their power no longer needs to pass through their Heart to be externalised. This means they can put out much more power at once, as there's no longer anything 'blocking' it and trying to shape it prior to release, vastly increasing their maximum output power (though not the overall size of their reserves) and robbing their spiritual energy of any signature beyond 'vast, overwhelming emptiness'.
The obvious problem with this is that it relies on the casting off of the Heart to achieve true Segunda Etapa being much more literal and final than I think it's meant to be. As in, it'd be a 'you can never revert from the state, and you lose all unique powers you ever had' sort of transformation. It's the best idea I've got, though.