Heel Flap
The heel flap is the rectangular, densely knitted section on the back of a sock heel, worked flat before the heel itself is shaped with short rows.
Explanation
The heel flap is knitted back and forth over half of the sock stitches, often in a denser pattern such as slip-stitch heel for extra strength and cushioning in a high-wear area. Once the flap reaches the desired length, the heel itself is shaped using short rows (turning the heel), before stitches are picked up along the sides of the flap to continue down the foot. A slip-stitch heel flap is one of the most durable heel constructions and is often preferred for socks that will see heavy use.