Will,
It is based on how your players perform that week as their stats are pumped into the SS computer and used to simulate games. A couple of things to consider. Forget rotisserie categories. Wins, Saves, Runs, RBIs and to a degree SBs are not as important in SS. You will want to look at how your players fair with OPS on offense and with your pitchers, ERA and WHIP are the key stats to drive your team. It is much closer to real baseball (defense also matters) than other fantasy options out there and you actually fill out a full lineup, bench and bullpen - where the 35 guy on your team can contrinute at times.
I highly recommend it.
John