Bobbin lead (pronounced “led”) is accomplished when the drive band goes around the bobbin groove, and the brake band goes around the whorl/pulley or flyer. The bobbin is “leading” the flyer, thus the term “bobbin lead.”
As you spin, and keep tension on the yarn, both the bobbin and flyer rotate together. When you release the tension on the yarn to allow it to wind on, the flyer will stall or stop and the bobbin will continue to turn, thus winding on the yarn.
It is also known as Indian Head Tension, Irish Tension, and German Tension, though ‘Bobbin Lead’ is a better, more descriptive term for it.