Last year, the 49ers defense was going to start the year out as a hybrid defense with the signing of Justin Smith. Having him as the key signee, they were planning on switching from a 3-4 alignment to a 4-3 alignment, which would move Smith from defensive end to sometimes an outside linebacker.
This was former coach Mike Nolan's plan, so they could change schemes whenever they needed to in games and make it hard for opposing teams to attack them.
The problem with this was the defense started the year in the lower rankings of the league in every defensive category, and the team didn't win many games either. This led to the firing of Nolan and the hiring of Mike Singletary.
After Coach Singletary took over the team, he made the defense permanently run a 3-4 alignment. This made it easier for the defense because they didn't have to switch alignments anymore. This made it simple for the defense and they ended the season ranked 13th in total defense.
This year, the defense is going to run the same scheme they were running at the end of the season and hope to continue the success they had.
This philosophy of staying with a 3-4 alignment should help the 49ers defense become a top 10 defense this year.