Czech Republic international midfielder Jarosik, who is on a season-long loan from Chelsea, pounced after 61 minutes to end Torquay's hopes of pulling off an upset.
Finland striker Forssell, making his first start since October, then made sure of victory with his third goal of an injury-hit campaign nine minutes from time.
Birmingham were grateful for the two second-half strikes as they struggled to see off the challenge of Torquay who are 71 places below City in the league structure.
While they dominated possession, the hosts lacked a real cutting edge in front of goal and when they did hit the target found Torquay goalkeeper Andy Marriott - a former Birmingham loanee - in impressive form.
Torquay, who had come close to beating Birmingham in their first meeting at Plainmoor, were far from overawed by the challenge.
If their finishing had been better they would have taken the lead, with Kevin Hill wasting their best chance from close range in the 33rd minute.
Birmingham's Premiership class eventually told in the end but only after a monumental struggle.
As expected, Birmingham quickly laid siege to the Torquay goal in the opening stages but were frustrated by the woodwork in their bid to take an early stranglehold on the game.
David Dunn saw his 10th-minute header saved by Marriott and Jarosik flashed a shot wide before the post came to the rescue of the Gulls.
Julian Gray fed a 25th-minute ball into the area and Forssell spun away from Torquay captain James Sharp and beat Marriott with a low drive, only to see the ball crash to safety off the post.
Birmingham struggled to come to terms with their bad luck and the let-off inspired Torquay to their best spell of the half, which should have seen them take the lead.
Giant midfielder Morike Sako was twice denied by smart saves from Maik Taylor when he let fly from long range.
But it was Hill who should really have broken the deadlock when he ghosted into the area. He was unmarked and just ten yards from goal when he met Steve Woods' corner with a first-time volley.
It looked easier to hit the target than miss but Hill fired wide before reeling away holding his head in his hands, well aware of just how good a chance he had missed.
That sparked Birmingham back into life and Torquay were again indebted to Marriott for keeping them on level terms when he produced a smart one-handed save to tip away a drive from Jarosik six minutes before half-time.
That set the tone for the start of the second period and Birmingham's pressure finally paid off on the hour after Jermaine Pennant, who had proved to be a thorn in the side of Torquay all match, whipped in another one of his trademark right-wing crosses.
Marriott looked to have cut out the danger when he punched the ball clear. But his clearance flew straight to Neil Kilkenny on edge of the area and when the rookie midfielder fired the ball back in, Jarosik was well placed to nonchalantly side foot it home.
Birmingham were unfortunate not to double their advantage a minute later when Marriott pulled off a stunning save to deny Dunn.
But he was powerless to prevent Forssell making the game safe in the 81st minute when Pennant once again proved to be the provider with another right wing cross that Forssell touched home from six yards.