The silver knob aligns the take up lever on the active needle with the arm that makes it go up and down.
The take up levers should all be in the same position when the needle is in the up and stopped position. You can check this by flipping down the magnetic cover on the front of the needle case.
If you have one that is slightly dropped when the flywheel in the back is set at 100 and a grinding noise when changing colours your take up levers may need adjustment.
You can do this yourself as it is in the manual but you will need to buy a long handled T allen wrench as the machine does not come with one. It can be accomplished with the tools supplied but not as easily as the T wrench.
The manual will show you which covers to take off to access the hex screw through the front of the head. Loosen it off with the levers in the up position and make sure the wheel is set to 100. Align all levers exactly and re tighten screw. Test machine by selecting each lever in turn. It should change smoothly as they ride on the rail.
It is easier than it sounds. Like a jigsaw puzzle... one step at a time. Lay the parts out in order as you take them off and put them back in order. SWF has step by step with photos for most maintenance tasks. Just take your time. If you get stressed take a break and walk away for 10 minutes.
