Another type that qualifies under "cannot slip; I.e. it is either
engaged or disengaged" are seamless gear boxes which sports the
advantage of the input shaft and gears are essentially one unit, with
the gears splined to the shaft and spinning at the same speed at all
times. On the output shaft, all the gears spin freely, but inside each
gear is a set of four pawls that can lock the gear to the shaft to
transmit power. The pawls are pushed into place by pins that are
extended outward from the center of the shaft by a set of rods inside;
using a ratchet effect, two of the pawls lock into place under power,
while two lock into place under deceleration.
Honda's patented 50K seamless shift...