The bridging stitch is #10 on my Husqvarna Lily 550,
and #31 on my Janome 6600.
This outs me as an extravagant seamstress owning multiple sewing machines, but the Husqvarna officially has a completely worn out motor, which my sewing machine repairman tells me is due to my non-domestic level of use of the machine. Doesn't he know that sewing
The stitch zig-zags alternately on either side of the needle. In addition to shirring, I use this stitch for fold-over elastic application on underwear and sportswear.
Once upon a time when I dabbled in heirloom sewing, I also used it to attach laces to each other. My Husqvarna manual says " The bridging stitch is suitable for joining two pieces of fabric with folded edges. Leather and vinyl are joined by butting the edges close to each other using the bridging stitch. Also use to join lace and ribbon."
The real benefit of using the Husqvarna for shirring is the buttonhole foot, C. This foot has a small hook at the back for corded buttonholes, and also has a slight groove under the inside edge of the place to hold the cord arms away from each other.
This means you can sew over two strands of elastic at once, without much danger of catching the elastic in the zig-zag and ruining the shirring, as the elastic is nicely held in place by the foot, provided you hold the elastic in front of the foot under a little tension.
Unfortunately, I do not have an equivalent foot for my Janome.
Once I have sewn enough strands of shirring, I have a short cut for distributing the gathering.
I pull up the elastic quite aggressively at one end.
Then I hold the elastic threads firmly at both ends, and pull the fabric firmly side to side.
As the elastic stretches then relaxes again, the gathers distribute reasonably evenly over the whole piece of fabric. Any remaining bunching can be sorted out by repeating the process, or stroking the gathers apart or together as needed to provide even gathering across the desired width.
At this point I use very narrow zigzag to stitch vertically across each end of the elastic threads. I then tie off both ends of the shirring.
I find the tying off necessary so that I can wash the finished garment in the washing machine. The loose ends of the elastic tend to fray, and the knots keep the fraying from intruding into the shirred area.