The safe can get jammed on few numbers and then there will be. I hope that helps, I know overlockers are amazing machines and I am sure you will enjoy using yours.
#Brother Lock 640D Instruction Manual manual#
The manual might be useful for you, but the one I found has to be paid for unfortunately and will cost $10 to download.
The machine that the lady is threading is not the same as yours, but I think you will find it similar enough to be helpful, at least I hope so. I find that it is easier to actually watch someone and learn from that, than to have someone explain it as that can get confusing. Threading a Brother 760d overlocker looks tricky, so I have included a video of someone doing it.