"We had a few tips off of film that we were able to differentiate between run and pass early. That kind of gave us an added benefit on defense," Dahl said before the second day of the team's mandatory minicamp Wednesday.
The talented and tough Rams' defense combined for seven sacks over the two games and held the 49ers to just 13 points in their loss in St. Louis Week 13.
That performance came after the 49ers had put up 32 points against the Bears and 31 in a win in New Orleans in back-to-back weeks.
"It was some personnel and some alignment stuff that were the giveaways," Dahl said.
Since arriving to the organization March 16 signing a three-year deal with San Francisco, Dahl said he has communicated the giveaways to his new coaching staff.
"It's been addressed and corrected. They knew most of it before I got here," he said.
Dahl joined the 49ers to help fill the void left by All-Pro safety Dashon Goldson's departure in free agency. The two-time Pro Bowler signed a five year, $41.25 million contract with Tampa Bay in March.
The free safety position remains a question mark for a defense that's been one of the league's best since Jim Harbaugh and his staff took over in 2011.
Top draft choice Eric Reid is likely the long-term solution, but the team has allowed Dahl and C.J. Spillman to work with the first team so far in minicamp. Defensive coordinator Vic Fangio said Reid will run with the first team defense Thursday, the final day of minicamp.