Spining the body during meditation – Is it OK?

There is nothing “wrong” with what is happening.
The body will simply do what it feels it needs to do,
to accommodate the energy flow that you are experiencing.
This is an example of what is called “spontaneous qigong”.

It seems to me that the reason why the energy is
making your body move is due to its intensity,
perhaps amplified by your retention practice.
The intensity is likely more than the body
is prepared to deal with, so it is taken over
and moves spontaneously in response to it.

The way to be able to handle an increased load
of energy, to be able to digest it, to integrate
it in to your body . . . is to get more deeply
grounded. I would advise more physical body
practices, especially standing/rooting practices
like Iron Shirt 1. If you are doing sexual
retention practices, then you need to balance
this out by doing extra grounding . . . definitely
more Iron Shirt 1 and things like Deep Earth Pulsing
(from QF3 and Sexual Vitality DVD).

Other grounding practices that make you
more deeply embodied such as Tai Chi and
movement qigong would all be beneficial
and supportive I feel.

Actually, all the advice above not only will address
your question relating to handling the increased
energy load, but it will also address your question
as to how make the pearl have more substance
and how to give more control over its behavior.
Grounding practices and body movement practices
such as movement qigong help you to become
more embodied, which will make the alchemy (Fusion)
meditations you do more physically substantial
and will allow you greater control over the
whole energetic makeup.

Qi,
Steven