Sometimes, if you're working with an older sewing machine, your bobbin winder won't disengage, and the needle will go up and down while you're trying to wind a bobbin. This can be caused by 2 things. #1) you've got some old oil on the shaft inside that needs to be removed. Or #2) the ring under the bobbin winder clutch is in the wrong position. Here's how to fix it...
Here's a picture of a bobbin winder clutch. It's on the hand wheel of this old Kenmore. First, loosen the small screw on the center knob. You don't have to take it all the way out, just loosen it. Then twist the center knob to the left (righty-tighty, lefty-loosey) until it comes off. This is a picture of what you'll see under the knob. Can you see the metal ring that's sitting on the center shaft? Remove it.
You may need to take the top lid and/or the side panel off to allow the hand wheel to move in and out. If you're not sure you can get the belt back on correctly, you don't need to take the belt off, just loosen the outer panels so you can move the hand wheel on it's shaft, like this...
Can you see how the center shaft has moved back inside, as I push the handwheel out? Use some "solvent" (WD-40) to remove the old, gummy oil on the shaft. Squirt it inside the hole and work the handwheel around until the gum is dissolved. Then wipe out as much of the solvent as you can. If you have a can of air, you can blow it out -- we use an air compressor at the shop. Also use the solvent to wipe the old oil off of the metal ring and center knob.
Put 2 drops of oil on the shaft and put the machine back together.
The metal ring under the knob will have 2 or 3 raised nubs on the inside.
The raised side of the nubs should be facing outward. Put the ring in place, with the nubs in the slots on the shaft. (Take a closer look at the second picture above.) Then screw the center knob back on and tighten the little screw. You should be able to release and re-tighten the center knob. If not, remove the center knob and re-position the metal ring. Turn the ring clock-wise one position (or two). Screw the center knob back on and tighten the little screw.
That should take care of it!
I neglected to mention that the bobbin winder spindle can become frozen as well. Here's a couple pictures of a Singer bobbin winder.
If it's hard to turn the top spindle by hand, it's probably frozen-up by old oil. If it's frozen, you can try two things. First, I usually put some WD-40 in the cracks (top and bottom) and work it until it's loose. Always blow the WD-40 out with an air compressor or canned air. Then re-oil. If that fails, you can disassemble the bobbin winder, clean it, and put some oil on the shaft directly. Then put it back together.