Arm recoil is a neurological examination of neonate for detecting the muscle tone.
Arm recoil Wikipedia
Baby supine, forearm flexed at elbow. The elbow (forearm) extended by pulling the hand; then released.
How quickly the forearm returns to flexed original position.
More the tone (flexor tone) development more brisk will be the recoil.
Following the same principle leg recoil can be tested.