In one of Scott Kelby's books he says that using 2nd curtain will help keep the background from going dark. He even has a rather dramatic set of pictures - the first was 1st curtain and it had a very dark background; and the second used 2nd curtain (and he said that was the only change) and the background came out well lit.
I do not get it myself... so I tried to duplicate his results with a subject with a dark background, but every shot looked the same (looking on the LCD display) whether I used 1st or 2nd curtain.
I guess I still do not get it. Same exposure, just different sequencing of the light should not make a difference in the outcome.