Feeling that my belly get sucked in

Nice session guy. Sounds like a major step forward on your journey.

Many Aneros users encounter their initial strong feelings of sexual energy in their lower abdomen. There are multiple labels for this region which are each defined in various (mostly Eastern) scientific disciplines such as:
-- Yoga & Stationary Meditation
-- Martial Arts / QiGong / Moving Meditation -- with sub-specialties such as Tai Chi, Tai Chi Chuan, etc.
-- Traditional Chinese Medicine (TCM), dealing with the flow of Chi along Body Meridiens and the application of Acupuncture or Acupressure to achieve pain reduction and/or healing.

I usually refer to my Navel Chakra as "Dan Tien" which was used in a Military Hand-to-Hand instructor years ago. It is near the human body's Center of Gravity and a postural center for the application of the most damaging hits. Belly breathing during fighting, meditation or sex best employ some sensory reference to one's DanTien.

A few of us build the strength of our session p-waves through gentle massage (sometimes with only proximity of a hand -- Reiki massage) of this area. -- Google for "Male Deer Exercise."

As an Aneros junkie I usually generate energy in my prostate, attract it to my Navel Chakra (Den Tien) then further attract it to my Heart Chakra through nipple manipulation. Curiously my a-less (uninserted) sessions start with an energy spike in my Heart Chakra which I attract downward using Male Deer moves.

If you are interested in a more thoroughgoing explanation, I suggest studying Mantak Chia's description of the "Microcosmic Orbit" which is his basis for Multiple Male Orgasm and energy exchange between sexual partners.

The easiest way to get into Chia's terminology and labeling is his small volume, "The Inner Smile."