I am struggling to learn the bossa rhythm for the song Black Narcissus.My version of this song has a B flat 7 flat 5 chord in the “4 and “ position of bar one. This chord repeats in the first two beats of bar 2. I understand that this is called a push chord or note. It sounds good but why does it work?
"Why does it work" - the more general phenomenon behind it is syncopation. https://en.wikipedia.org/wiki/Syncopation
Syncopation means playing something off-beat, and in order for that to work for you, you have to have a strong sense of on-beat. https://en.wikipedia.org/wiki/Beat_(music)#On-beat_and_off-beat
How to make it easier to play? By developing your sense of on-beat. This is largely hypothesizing, but I think the most important thing is to train yourself to produce the on-beat rhythm at all times, instead of just identifying it by listening to played on-beats. An exercise for this is to let a metronome play the twos and fours in a slow 4/4 rhythm, so you have to produce the "one".
Synchronize yourself to the twos and fours, and play different songs and rhythms on top of it, so that your one is on the one. Not on the twos and fours of course.
Anticipation. We expect the first beat of a lot of bars in pieces to be the strongest - that's often how we determine how many beats are in the bar anyway!
So we expect a chord change on the first beat of a bar, but with push chords, they come earlier than expected. It sort of moves the song on, sooner than it would if the chord changed on the next bar. It doesn't even need every player to do this - in fact, it sounds fine for say, just the chord player to push.
The Beatles, amongst many others, did it in quite a few of their songs aand I guess it's a hallmark of 'jazzing up' a tune. Generally using the last part of a bar - the & of 4, or even the 4 itself.
There is a close relation between music and physical movement, especially between rhythm and dance. This seems obvious and scientific research suggests that for humans it is a natural thing. (Search internet for something like "music and children" and, in any case, read Musicophilia by Oliver Sachs). I think it is key to understanding what makes your "push chord" and many other forms of syncopation interesting.
Imagine you are dancing to the music, putting your feet down on the count. If you can't imagine youself dancing, walking will do just as well. A strong accent between the count, like your push chord in the bossa nova, will lift your body up since at that moment it is in the air moving from one step to the next. Can you feel it? It applies to many kinds of syncopation. The chord on the "4 and" you describe, gives me the feeling to almost stumble and just in time find my balance again.
Even if you are not moving it makes the music more exciting because of the natural connection between rhythm and physical experience.
It works because it advances or resolves the melody faster than expected/needed, which is exciting! A lot of the pleasure we derive from music is from pattern recognition at first to learn the general structure, but also by being pleasantly surprised by creative deviations from that structure.
Besides "pushing the chord," I've also heard this referred to "anticipated bass," or the "tumbao" rhythm in Afro-Cuban music which is usually notated as 2 notes per bar, on the and of 2 and the 4: https://en.wikipedia.org/wiki/Tumbao