From the area around your naval to the root of the body (your asshole) is this primary cultivation space. Here is your pelvis and lower back, lower abbs, genitals, guts, bladder, etc.
Usually I focus around my lower back and belly region to feel it and make sure to spread out until my hips come into sensation.
If you focus here with a mildly firm but relaxed posture it massively increases vitality, rests anxieties and allows you to build up physical energy and store it.
Food for example is digested and will restore this regions energy which is depleted from physical work, stress, sexual activity and other takers.
The best foods for restoration of this space is anything rich in energy, fat and protein. Heavy meats and organs(liver, kidneys), exotic sea foods like eel, dairy, oils and root vegetables.
Good herbs are those not heavily stimulating and good for nourishing yin and the kidneys. Ginseng while stimulating is good, fennugreek seeds, yam root, horney goat weed, donguqai and noni fruit root to name a few.
Physical exercises that aid this region are Deadlifts, anything for the legs, Abdominal and oblique exercises, Shiko (Sumo leg lifts), Walking, Sex or sexual exercises that move the hips and lower back, Lower back and leg stretches, Horse stance (Standing with legs mildly bent) and Kegels that also use the lower ab regions.
Spiritual exercises
Sit where comfortable but will not fall asleep and relax your body and head. Take a moment to focus on your breathing. After this center your focus around the lower energy center space, make sure you can feel your lower belly, lower back and pelvis.
Keep a tensed-relaxed sense of focus here, as if there was a half sized basketball full of energy. When you breathe, breathe deep into this space.
Continue this to build up a sense of weight. When you are ready, pull in lightly with your entire body, only using the subtle energetic-mental force and not your muscular or physical force and guide it all into this lower space as if it were an energy vaccum cleaner.
This will cause a lot of pressure to build up so do be careful how much energy you take in.
If you've taken in too much or need to release it then spread out your focus to your whole body and allow the built up pressure to flow smoothly throughout it.
Another post is planned to go further in depth about esoteric sexual practices but it will be spoken of here in relation to the lower energy center.
When we perform any sexual act (alone or with ourself) the lower center is considerably stimulated and exercised. New blood will be brought into this area and hormones generated.
This creates a lot of rich blood and energy that circulates through our body and aids it in recovery, etc.
Through proper sexual practices the lower center is invigorated and through improper or harmful ones the energy is expended and musculature is harmed. Proper could be warming up, control of the energy flow (making sure there is not excessive pressure or blocks), happy mood and emotions, unstressed movements, etc. Harmful can be excess far beyond ones limits, the over use of kidney yang stimulants like erectile tonics, strained movements, outside stressors (loud sounds, smokey air or bad smells), disharmonic relations with another or internal sexual trauma, etc.
Ejaculation is another big topic of discussion for occultic sexuality.
Retention of ejaculation is only possible in "Pure Yang" cultivation styles. These are ones that create immense heat and pressure within the body, hence "cooking" your ejaculatory fluids upwards and is not recommended for common folk as it can cause kidney stagnations and other sexual health problems. Though over ejaculation can cause kidney yang deficiency in men and kidney yin (moisture and liquid) deficiency in women.
So with that, know your own limits and make sure to regain your energies.
Do note that a weak and/or exhausted lower center can cause difficulty in erection quality and lubrication. Also difficulty moving the lower back.
Always making sure that you've had enough sleep, keep a restful lifestyle, are eating well, am sexually healthy and not stressing your vital force is key to keeping the lower energy center healthy.
*Disclaimer* This is not medical advice. I am not a medical practitioner.