Signing activates a series of joints along the arm and hand from the shoulder and elbow to the wrist and knuckles.
As infants learn to control joints of the arm, they acquire a new motor skill starting from the shoulder to the elbow and down to the details from wrist to knuckles of fingers over a period of time.
In this process, infants often sign in bigger movement patterns from the shoulder (the joints most proximal to the torso) and gradually to the wrist and knuckles (the joints most distal to the torso).
The video above shows an example of proximalization of movement in a child whose native language is American Sign Language (ASL).
For information on proximalization of movement in signing specifically ASL, see studies done by reserachers Mirus, G., Rathmann, C., and Meier, R.P.
You may be interested in language acquisition from birth to preschool.Manual babbling in sign language.