Row 1 (RS): SL1, *K1, SL1*, repeat from * to * until 1 stitch remains, K1. Row 2 (WS): SL1, *P1, K1*, repeat from * to * until 1 stitch remains, P1. Repeat rows 1 & 2 until heel flap is desired height.
As you can see row 1 follows the standard heel stitch pattern, but it changes on row 2 (the key to the extra padding). On row 2, you knit the knit stitches and purl the slipped stitches (from the previous row). This results in a column of garter between the slipped stitches, which provides the additional padding. The garter column causes heel stitch to contract more vertically than a traditional heel stitch and you may need to account for this when using it in a sock.
The end result is a heel which looks like a normal heel stitch, but provides extra padding. If done in a lighter color the purl bumps may become visible.
Photos copyright Prairie Willow Knits..