In the centre of the hand wheel there is normally a knurled knob. If you hold the hand wheel firmly you should be able to rotate the knurled knob by about 1/8 turn. This engages or releases the clutch and enables the motor to be run without moving the needle. It SHOULD be used when winding bobbins (my mum called them spools) for use under the plate for the lower thread. She was taught (and she was a professional machinist) that winding the bobbin with the needle operating was not good for the machine.