SINGER 14T968DC - Manual (Page 21)

Install the Knife Cover.
Engage Upper Looper. Engaging the Upper Looper causes the looper to move
when sewing. Even though the Upper Looper is not threaded, it needs to be
engaged.
1. Push the clutch button to the left and down to engage.
2. Turn handwheel.
Upper Looper Disengaged
Upper Looper Engaged
Two-Thread Wrapped Edge Overlock (narrow) page 5