Quote:
“Heroes” creator Tim Kring will oversee a second crew on the new show, which will explore the origins of various characters, some of whom could be added to the main show’s cast. The new series will air in the original’s Monday timeslot, most likely after the current season ends or during another hiatus as part of NBC’s efforts to reduce the number of repeats, for which viewership has steadily dropped off in recent years. “Heroes” itself was picked up for a full 24-episode second season.
Source: Variety
Quote:
In an American Idol-esque twist, viewers will get a chance to pick their favorite new superhero during the spinoff’s six-episode run, with the character tallying the most votes moving up to the big leagues, becoming a full-time cast member on Heroes during the 2008-09 season.
Source: E! Online
It seems that the spin-off is NBC's way of filling the gaps during the various hiatus that happen in the middle of the season. I'm not sure why they don't just add more episodes of Heroes itself, but more Heroes related content is good content in my book.