Trembling of the body during pranayama is known as kampana.
This is experienced by a practitioner of yoga in two different ways. Patanjali
has mentioned in Yoga Sutra (I.31) five situations which arise in the wake of
the nine obstacles on the path of Yoga. They are – pain, mental dejection due
to frustration of desire, trembling of the body, forcible inspirations and
forcible expirations.
The trembling of the body parts caused by lack of stability
is a situation which puts an obstacle in the practice of posture for sitting in
meditation. To overcome all these five situations Sage Patanjali has
recommended the practice of pinpointedness (Yoga Sutra I.32).
The other way of experiencing trembling is associated with
prolonged practice of pranayama. It is described in Hatha Yoga Pradipika (II.
11-12). It is recommended by Swatmarama that a sincere student who wants to
derive the full benefits of pranayama should practice it four times each day.
And at each sitting one should practice 80 kumbhakas, that is holding the
breath inside the lungs.
The duration of each kumbhaka is measure according to three
standards. The lowest standard means holding the breath for approximately
twelve seconds in each kumbhaka; the middle standard is of kumbhaka for twenty
four seconds, and the highest standard includes each kumbhaka of forty seconds
or more. Practicing three hundred and twenty kumbhakas of the lowest measure
every day leads to profuse sweating of the body. The middle standard gives rise
to tingling sensation along the spine. This is an indication of good progress.
Source – Encyclopedia of Hinduism Volume V page 439 - IHRF