I had the same question about locking a stitch down when- bringing the bobbin thread up to the quilt top while quilting..Then I found a youtube vid on Cheater Needles, under ( how to tie a knot in thread for hand-sewing or quilting)..
It looked soooo easy to do...I got a pack at DaySyleDesigns(dot)com
Sorry if I'm preaching to the choir..Very much a newbie here...But figured it could help someone out there lol