1) The band-limited almost-subliminal vocal in the background seems to be almost a must-have on a pop vocal track anymore, and you'll find it a lot in Genie right from the beginning.
2) The vocal effect is pretty nice. It's a slightly delayed reverb combined with a medium length delay that hangs over at the end of phrases, which fills up the spaces nicely.
3) The pre-chorus harmonies switch to a longer verb and longer delay that's makes the part a bit eerie. It's a great example of using effects to set a mood.
4) The stacked harmony vocals in the chorus, on the other hand, are fairly dry. Christina sings very well with herself and all the vocals blend extremely well. As a singer, you need a sophisticated ear to do this and you can tell that she has one.
5) The ad lib vocal in the outro is mixed a bit back from the harmony vocal hook line in a lot of places, which is somewhat backwards from the norm. That strategy works well for this kind of music and certainly works here.
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