loose end finish by burning or melting the loose ends
Natural Fibers
Burning natural-fiber thread typically works to shorten it, similar to trimming flush with the stitched material. For easily unraveled machine stitches, such as lockstitch and chainstitch, this can create a high risk that the final stitch will come loose and begin to unravel. As a result, burning isn’t recommended, but the risk can be reduced with backward stitches.
Synthetic Fibers
Burning some synthetic-fiber threads melts them rather than burning them away. Mechanically, this can create a stopper wider than the last hole of stitching, which acts to stop the thread from coming loose by feeding back into the hole, similar to a knot when tied off.