The Take Up Lever

The loop of thread under the cloth must be tightened after the shuttle has made its pass. Sometimes, particularly in early machines, there is no take-up lever. The thread just goes through a hole at the top of the needle bar. As the bar is raised the thread is pulled upward and the loop tightened.
A take-up lever which fits loosely through a slot in the needle bar makes this action more positive and can also advance the action depending on the position of the slot.