The principle of selective reinnervation is to use the ipsilateral proximal facial nerve to power eye closure and the ipsilateral masseteric nerve to power the smile.
Smiling and biting are synchronous actions. Patients can learn to perform both activities naturally at the same time. This is a distinct advantage over other nerve input options, such as hypoglossal or spinal accessory.
With ongoing practice and physiotherapy, some patients can learn to smile spontaneously. This may occur due to cortical adaptation.
General considerations regarding nerve repair techniques can be found here.