Franchise players are paid their salary via 16 prorated game checks. As such, they can sit out the entire preseason without being fined/losing money. Once the regular season starts, they lose 1/16 of their salary for every week/game missed. The week 11 deadline is for a service year, which, if not met, would affect Bell from receiving the higher, more prohibitive, franchise rate.
Bell will miss the preseason and show up for week 1, like he did last year. Sitting out a potential playoff game (or games) is interesting because he would only miss the bonus playoff check(s), which is quite small compared to his regular salary. Also, I believe the extension window re-opens after the regular season, so he could possibly extort the Steelers into giving him an extension with guaranteed money before playing in the playoffs if he wanted to really push it.