The Die Collet holds the die on the edges. This tool is made in tungsten carbide, the inner inverted pyramid walls will help self-centering the die during the pick-up phase. This geometry allows a scrubbing action, necessary for eutectic alloy die attach option. Die collet may have only 2 walls contacting the die, the other two sides being open. There is certainly a vacuum loss in that case but the force is still enough to hold the die without any consequences.