Sentence:  Briefly, in the invention, the shift mechanism is incorporated into the handlebar stem, a positive push-pull type cam mechanism is provided for actuating the rear derailleur, <WSD>undershift</WSD> and overshift is prevented to eliminate chain "jump off", and the device can be shifted with the bicycle standing still. 
Meaning of the word marked with <WSD> tags:  (mechanical engineering) A misalignment resulting from failure to shift gears far enough.