Knight's razor sharp finishing cut Chesterfield to ribbons and gave Swansea a marvellous end to the league campaign and sent them into Thursday night's semi-final first leg with Brentford full of confidence.
It was a brave decision by Jackett to put Swansea hero Trundle on the bench but by the end the 2,000 travelling fans were singing Knight's name after he scored three times in a thoroughly impressive team performance.
The former Brighton striker was a constant menace alongside Rory Fallon who headed the second goal which gave his team a cushion to go on and win the game.
It might have been different if Colin Larkin's header had not come back off the post with Swansea only one in front but Chesterfield were second best in virtually every department.
They struggled to cope with the pace of Knight as Swansea knocked the ball long to get him in behind the defence.
A mistake by Aaron Downes let him in as early as the fifth minute and he drove a right-foot shot inside the upright to open the scoring.
Knight was shown a yellow card by Durham referee Nigel Miller for removing his shirt but that moment of foolishness was quickly forgotten when Fallon headed Andy Robinson's corner into the roof of the net.
Knight dragged a shot wide four minutes later and Swansea continued to dominate at the start of the second half with Barry Roche pushing away a snap-shot from knight in the 46th minute.
The goalkeeper then tipped over a Gary Monk header and pushed away a fierce shot from Robinson as Swansea pulled Chesterfield apart.
Larkin volleyed straight at Willy Gueret in the 66th minute but it was an isolated attack from Chesterfield who were finally broken again when Knight lobbed Roach from just inside the area with 11 minutes left.
The rout was completed in the 89th minute when Knight turned before steering a shot with the outside of his right boot around the keeper and into the bottom corner for a memorable hat-trick.