A variety of conditions and injuries can cause you to lose hand function. If you injure a nerve, the muscles “controlled” by that nerve will not function. If a muscle or tendon has been damaged or lacerated beyond repair, it cannot perform its function either. Some children are “born without” certain muscle functions. Some nervous system disorders like cerebral palsy or stroke can cause imbalance of hand functions. All of these are circumstances which might be able to be addressed by transferring a tendon from a functioning muscle to a new tendon to restore function.
Believe it or not, there are forty muscles below the elbow, all of which have different functions. During tendon transfers, only the tendon insertion to bone is changed; it can be attached to a different tendon or a different bone and then will perform a different action.
The tendon transfer will take time to heal, and splinting/casting and therapy to teach you the new tendon function will be prescribed. You will need to strengthen that muscle once healing has occurred.