|This is a well known trick for quilters, I'm sure, but I'll describe it briefly again.
1. Stitch a single stitch while holding the upper thread.
2. Pull upper thread out of fabric and bobbin thread should follow and poke up through the fabric.
3. Use fingers (or tweezers) to bring out a longer length of bobbin thread.
4. Hold both thread tails in 1 hand as you do the first few stitches of a seam.
The reason this is done is to avoid the thread nest on the underside of the fabric, which is useful for quilting. Not being a quliter, I'd not heard of this technique until recently. When starting a seam, I always just held the 2 thread tails together (1 on top, 1 on bottom) to stabilize the threads, per my manual. However, when starting a seam in the middle of a large piece (eg, bar tacks on a jeans fly), I was unwinding very long lengths of thread to be able to hang onto them. By bringing the bobbin thread to the top, I no longer need to pull out more than a few inches of thread.