here's an interesting article from the iaso record site on how a bachata song is formed - what each instrument does:

http://www.iasorecords.com/release/b...akdown-en-vivo

in particular, the requinto (or in my case, mandolin):

Requinto (Lead Guitar):

In derecho, what the requinto plays can vary greatly depending on where it is in the song. In the instrumental intros, the requinto will play a melody that is often loosely based on the singer’s melody in the verse. The melody played by the requinto in this section identifies the song to the listener before the singer has even begun to sing. The requinto plays a more distinct pattern in the verses. It has a string pluck on every beat and upbeat – so as with the güira and rhythm guitar is plays 8 times in a measure. The most common pattern would be descending from low notes to high in a chord, in two groups of three and then a group of 2 (total 8). In other words, it arpeggiates a chord: low, mid, high, low, mid, high, low, high. This pattern is unique to bachata – in bolero the requinto does not do this.