The mechanism by which the FSPR procedure relieves the spasticity of the limb of the child is based on experimental evidence from physiology: the downstream conduction bundle of the spinal cord has an inhibitory effect on motor neurons, while the posterior root fibers entering the spinal cord have an excitatory effect. We comprehensively adjust the muscle tone of the patient by treating the posterior roots of the spinal nerves so that the muscle tone of the spastic muscles is as close to normal as possible. The posterior roots of the L2-S1 spinal nerve can be selectively cut during FSPR to eliminate the intersegmental connection from the posterior roots of the lumbosacral nerve to the adjacent anterior horn motor neurons of the spinal cord, thereby attenuating the lower limb muscle spasm and improving the motor function of the limb of the sick child. One of the meanings of selectivity refers to the selection of spinal nerve branches with a low threshold for severance. After incision of the dura mater during the FSPR procedure, the anterior and posterior spinal nerve roots are carefully separated under a microscope or surgical magnification, and each posterior nerve root is divided into 4 to 10 bundles of small branches, and their thresholds are measured separately with an electrical stimulator. The small fasciculus is cut off. In general, the number of posterior root branches cut in FSPR surgery is limited to 50% to avoid hypotonia after surgery. In layman’s terms, the surgery is monitored by a multi-conductor electrophysiological monitor, and the patient’s abnormal nerve fibers are selectively partially severed, resulting in immediate relaxation of the stiff limb. After a period of rehabilitation, the patient’s lower limb movement approaches normal.