The first thing we hear is the choir when Arwen rides Asfaloth toward Frodo. The voices build by starting on one note, then adding another and then a third to make a full chord at :10 when she jumps to the ground. There is no sensation of a beat, which makes us feel like we are suspended in time and it's in minor which adds the sensation of sweet sadness.
At :18 strings are added which makes the sound a little richer in texture while she speaks to Frodo. The voices continue to sing while she helps bring Frodo back to the light. Notice they stop singing when she stops talking.
At :40 it's the instruments only while the hobbits discover that she's an elf. The instrumentation is a little thicker here which gives us richer sounding chords. With lower sounding instruments playing the melody. The harp is added at 1:00 with a slow arpeggio which gives us a sense of gentleness. Aragorn tries to talk Arwen into staying with the hobbits at the point but even in their argument, you can hear the tenderness that they feel for each other through the music.
At 1:28 she mounts Asfaloth and the high strings are suspended again with no sense of a beat. This helps prepare for...
The shock of a strong rhythm in the next section after she tells Asfaloth to "ride fast" which starts at 1:37.
The low strings and brass give a loud, low note while the strings are added giving us a dissonant chord with a big crescendo (gets louder). The percussion also starts to give us a sense of movement as she rides through the forest.
At 1:44 the cymbal is rolled and a horn now takes the melody played in 4 while the bassoons and cellos, I think, do the off beats. ONE e&a TWO e&a THREE e&a FOUR e&a. This sounds like a galloping horse. The horns do sforzandos (LOUDsoft) with a crescendo (gets louder).
At 1:56 there is a slow, yet strong crescendo with a faster tempo which adds suspense to the scene as you see the Nazgul racing through the trees. The percussion changes the stress of the rhythm here by doing, ONE TWO three four ONE TWO three four and then changing it again throughout this section. The cymbal is added here in the crescendo and adds to the fff (extremely loud) chord that's at 2:03. The percussion changes to ONE TWO threeandfourand ONE TWO threeandfourand. The horns climb upward rising the tension even higher than before as the chase intensifies.
At 2:31 the choir enters and sings the Nazgul theme as they come out of the trees and are fully visible. The percussion changes again to a very strong beat of ONE &aTWO &aTHREE &aFOUR &a which is the sound of a horse's gait. You can also hear the horn answer the choir at 2:35.
The bassoons and cellos, I think, are added again at 2:42 doing the off-beats – e&a. This section is extremely rhythmic with the percussion, bassoons, cellos and choir all playing/singing a different rhythmic pattern.
2:53 Then they come rhythmically together with the pattern ONE THREE ONE THREE ONE TWO THREE FOUR ONE TWO THREE FOUR gradually getting louder and louder with the cymbal added, which always makes me think of the river as she crosses it.
At 3:01 the horns play what sounds like the same tune starting at different times which gives the sense of controlled chaos while the Nazgul enter the river. Of course, the horns do not play in the movie at this point, I believe it is actually the string bass that play this so we can hear the dialogue.
The Nazgul start to cross the river at 3:07 as a horn takes up the melody and as it plays, the other horns stop. The strings continue to play to maintain the tension.
At 3:17 the strings and then horns enter to produce a dissonant and suspenseful chord. They produce a BIG crescendo at 3:19 topping at 3:21 with a cymbal crash which I'm assuming represents the flood?
The rest of the orchestra dies out after 3:21 but the basses continue so that we do not feel completely comfortable since Frodo is about to lose himself at 3:31 as the choir enters again very similar to the beginning of the track except that the strings are with them. They sing the same melody but it is a little denser with a lower harmony this time. It also doesn't resolve at the end since we do not know what Frodo's fate is, the music just dies out without a sense of completion. This makes us wonder whether Frodo will be all right.