Steep tapers tend to loosen at high speed, as their solid shanks are stiffer than the spindles they fit into, so under high centrifugal force, the spindle expands more than the toolholder which changes the overall length: That is, as the spindle 'expands' the toolholder tends to move deeper into the spindle in the z-axis which can cause the production of parts that are out-of-tolerance. HSK's hollow shank is deliberately thin and flexible, so it expands more than the spindle and tightens when rotating at high speed. Furthermore, the HSK holder is dual contact: It engages with the spindle on both the taper and the top of the flange which prevents axial movement when thermal growth and/or centrifugal force of the spindle occurs.