This provides greater suction on the flat surface; hence, making pressure incredibly low (-0.17 MPa, i.e. lower than vacuum).

http://en.wikipedia.org/wiki/Acetabulum_(morphology)

This precise mechanism of high flexibility even has a potential mechanical applications in robotics.Each sucker is a tactile sensor for detecting the surrounding. When the sucker attaches itself on an object, the infundibulum maily provides adhesion while the central acetabulum is quite free. This provides greater suction on the flat surface; hence, making pressure incredibly low (-0.17 MPa, i.e. lower than vacuum). This is why octopus grip is exceptionally firm. Then contraction of the radial muscle of the acetabulum causes detachment of the entire sucker.