Follow the steps in order for the best result.
In a sturdy 2-quart saucepan, thoroughly combine the packed brown sugar and cornstarch, ensuring there are no lumps.
Measure the reserved liquid from the canned pineapple tidbits. Supplement it with enough water to achieve a total volume of two cups.
Gradually whisk this pineapple juice and water mixture, along with the vinegar and ketchup, into the sugar and cornstarch blend in the saucepan until smooth.
Place the saucepan over medium heat. Cook, stirring continuously with a whisk or spoon, until the sauce visibly thickens and comes to a rolling boil. Continue boiling for one minute while stirring.
Remove the saucepan from the heat and gently fold in the drained pineapple tidbits.
To serve, keep the sauce warm over extremely low heat or in a double boiler until ready to enjoy.