It is a myoclonic or hypnic jerk.

The medical phrase is "myoclonic tremor". It is caused by either an injury or condition that affects the nervous system, literally the nerves. Myoclonic tremor is often from a spinal injury. Myoclonic refers to the frequency and quality of the tremor, e.g. fast, slow, rhythmic, etc. Myoclonic can be further distinguished as tonic or clonic, which refers to the degree of musle rigidity that occurs during the myoclonic tremor. Note that the words jerk, jerks, or jerking are offensive to a person who has tremors.

Myoclonic seizures are characterized by brief, involuntary spasms of the tongue or muscles of the face, arms, or legs. Myoclonic seizures are most apt to occur when waking after a night's sleep.

In drop attacks, the child falls suddenly to the ground. This may be because the legs suddenly fold up (atonic seizure) or stiffen (tonic seizures), or because of a violent jerk (myoclonic seizure) that throws the child to the floor.

myoclonic jerks often seen in eileptic patients