The process for sewing a bar tack is essentially to sew several long, narrowly-spaced stitches along the line of the bar that will be formed, followed by short stitches made perpendicular to the long stitches, through the fabric and over the bar.
To create a bar tack, one sews several extended, closely-apart stitches along the future bar's path, then makes shorter stitches perpendicular to these, passing through the fabric and above the bar.