"has another experienced 'violent' jerk in the midst of meditation &
does anyone know why it happens & how to handle it?"
This is a fairly common experience with meditators who have made a
little progress. Some people experience a spazm in the legs, some in
the stomach some in the face, back, buttocks etc. some get chills or
hot flashes or prickly sensations.
This is a consequence of the release of old tensions. The body is
connected to the sense organs and reflects your personal reactions to
sense stimulation in the form of tension. When you sit down, you
begin to resolve some issues (you can do this mentally, or you can do
it just sitting letting go of the tensions).
The issue is the fact that this jerking around is disruptive of your
calm or concentration. What you can do is to try and "get ahead" of
these phenomena. Pay attention and you will see that you can see when
they are about to occur. At this time try to release the tension in a
smooth, controlled way.