void on the underside of the insole, within the rib, left by welting, before bottom filling
The height of the rib and the material sewn to it creates an empty space within it.
Handwelted shoes often have shallower cavities than Goodyear welted shoes, due to the low height of the holdfast carved into the insole. However, any channel carved into the insole at the back of the holdfast deepens the cavity where it runs.