Bakody’s sign, also known as shoulder abduction test is used to test for radicular symptoms, especially those involving the C4 or C5 nerve roots.
A decrease in or relief of symptoms indicates a cervical extradural compression problem, such as a herniated disc, epidural vein compression, or nerve root compression.
Abduction of the arm decreases the length of the neurological pathway and decreases the pressure on the lower nerve roots.

Ref :Orthopedic Physical Assessment ,Magee