Why do loose threads on the back need to be trimmed?
First, if there are several threads, or a ball of dog hair, on the back of the quilt, it can get stitched into the quilt and cause lumps to form. Secondly, a single thread of a dark color can show through a light background and distract from your work.
What is the purpose of stay stitching?
Seams that are not secured come apart on the long arm machine. When you stay-stitch around the perimeter of your quilt, you keep the seams together.
Why does the backing have to be 6 inches bigger than the quilt top?
This allows us to pin the backing on the machine (top and bottom) and attach the clamps (side to side) which keeps the backing taut while quilting.