This is a little trick I use when I run out of top thread - if the top thread has run out make sure that the old thread is still held in place by the tension discs before putting on a new reel. If you carry on sewing after the old thread has run past the tension discs, the tension of the stitch will be fouled up.
This is also a handy dodge if you want to change the colour of your thread as you are sewing.
These photos show how I have changed from light to dark thread to match the colours I am sewing over, keeping the work in the machine and without any interruption to the stitch underneath, so the bobbin thread just carries on as normal. The line of stitching which runs from the top to the bottom of the picture was sewn all in one go.
When I reached the end of the white material that I was sewing over, I made sure that the last stitch was still on the white and that I did not carry on sewing over the dark red.
Then, keeping the foot down and without moving the work, I raised the needle and pulled the top thread loose from above and cut it with about four inches of thread to spare.
Next, I re-threaded the machine with the dark purple thread, pulling the tail ends of both threads out of the way behind the foot, and carried on sewing.
To finish, the tail ends need to be threaded through to the back, and there is now an uninterrupted line of stitching that has changed colour exactly where you wanted it to.