By Brady Henderson

Heath Farwell figured there would be an opening.

While studying the 49ers' punt team, the Seahawks' reserve linebacker noticed one player's tendency to abandon his blocking assignment a little too early.

"We saw it on film," said Farwell, whose blocked punt set up a go-ahead touchdown that gave the Seahawks a brief fourth-quarter lead in their 19-17 loss to San Francisco on Saturday.

Heath Farwell's blocked punt set up a go-ahead score. (AP)

The 49ers, having dominated much of the second half, led 16-10 when they lined up to punt from their own 38 with about seven minutes remaining.

Farwell had seen that reserve linebacker Blake Costanzo, who lines up as a left tackle on the punt unit, was often quick to disengage from his block and head down the field toward the returner, leaving an unobstructed path to the punter.

Farwell, whose job on the punt return team is normally to slow the opposing team's tackle at the line of scrimmage, took advantage of that tendency.

"He's a guy that likes the quick release and tries to get out ..." Farwell said, referring to Costanzo. "We practiced it all week. I would kind of flash my hands (acting like) I was holding him up -- I'm primarily a hold-up guy. So I got an opportunity to kind of flash my hands and things held up and I went around him and it worked like the way it did in practice."

The Seahawks took over at San Francisco's 4-yard line. Marshawn Lynch took a handoff on the next play and found the end zone to give Seattle a 17-16 lead.

"It was an incredibly clutch play to make," coach Pete Carroll said of Farwell's block. "He did it perfectly, he timed it exactly the way he wanted and he made a play that an all-pro type of guy makes. It was big time."

Farwell made the Pro Bowl as a special teams player following the 2009 season. He spent his first six NFL seasons with the Vikings, who cut him at the end of training camp in September. The Seahawks signed Farwell on Oct. 19, and he has been a special teams standout for Seattle ever since.

The blocked punt was the first of his career.

"It feels good. I just wish it has been the game-winner. It would have felt amazing if we would have won the game because of it," he said. "But that's football."