An NBS freewheel is conceptually similar to a roller bearing, except that the rotational motion between the two composing rings occurs exclusively in one direction, i.e. it is one-way. This type of function is made possible due to the special design of the rolling elements themselves, or the raceways in which they reside, which in the predetermined "free" rotational direction do not oppose the relative motion between the outer and inner rings; on the other hand, as soon as the rotational direction of these elements is inverted, the rolling elements oppose this variation and lock the system. When the system is "locked", the freewheels work as a friction element, allowing the transmission of drive/breaking torque between the shaft and the housing. 
There are essentially two types of freewheels: sprag type (DC, RSBW) and roller type (HF, HFL, NSS, NFS, AE, NFR, etc....).
NBS freewheels, both sprag and roller type, have three main uses: overrunning, indexing and backstop.