Dan,
At the time of the Civil War, blackpowder filled cartridge bags were often tied onto the wooden sabot for round shot or to the wooden base of Cannister rounds. This was done using a string or twine. The Sabot had a groove around the bottom & the string forced the material of the powderbag into the groove to secure it to the ball-This was termed "Fixed" ammunition.
Sounds as though the "simple knoting" was basicly a way to tuck the ends of the twine into the groove to keep them from getting accidently pulled or wedgeing under the ball as the round was being loaded.