Abstract: An unwinding apparatus has first and second pre-stretching rollers and a motion transmission system for transmitting a rotation motion from the first pre-stretching roller to the second pre-stretching roller. The motion transmission system has two toothed driving wheels rotated by the first pre-stretching roller, two toothed driven wheels selectively couplable to the second pre-stretching roller, and a selector device operable to couple a toothed driven wheel to the second pre-stretching roller. The selector device has a driven shaft provided with a longitudinal internal cavity having first and second lateral through openings suitable for housing respective locking elements. A selector pin provided with first and second transverse protrusions is movable inside the internal cavity between a first operative position, in which a first toothed driven wheel is coupled to the driven shaft, and a second operative position, in which a second toothed driven wheel is coupled to the driven shaft.