HSK Tool Holders have a short, hollow taper that contacts the spindle on two surfaces. HSK tool holders provide improved rigidity during HSM operations. The holding mechanism for HSK tooling is placed within the (hollow) body of the tool and, as spindle speed increases, it expands, gripping the tool more tightly with increasing spindle speed. There is no pull stud with this type of tooling. HSK tooling is sometimes called "Hollow Shank Tooling".