7 Series (black bobbin) bobbin winder jig is used to easily make adjustments to the amount of thread that is winded on your bobbins.
To use it is simple. Put the bobbin winder jig on the bobbin winder. Turn it to the arrow that allows the winder to engage (machine power must be off) then twist the bobbin winder to the other arrow and it should disengage at this point. If it does not you need to make the adjustment by inserting a t10 driver into the hole next to the bobbin winder.
This jig may be used on other machines(new generation 4 and 5 series, Black Bobbin) , but would require a Repairman to make the adjustment.