The create a yarn/thread shank technique is used to secure a button to a garment, while also creating a button shank.
A button shank provides a small amount of space between the button and the garment so that slipping the button through the button whole is easier.
How to create a yarn/thread shank
Anchor yarn at button placement (either on reverse side or at base of button). *Thread yarn/thread through shank or buttonholes, anchoring yarn at base of button and pulling yarn/thread so that button sits needed depth from fabric. Repeat from *, ending on reverse side. Anchor yarn. Secure backing button.