What Mantak Chia is talking about is Bone breathing and/or bone compression breathing. You should NOT try this until you have completed grand circulation and are comfortable with skin breathing and 4 gates breathing.
Basically in the first method, bone breathing, first imagine you are nothing but bones. you should forget about your skin, muscles and internal organs. you breath in and imagine qi entering your fingertips and toes, imagine chi entering your bone marrow one body part at a time until it feels full (ie hands, wrists, forearms, upperarms, feet, ankles, calves, knees, thighs, hips, spine, shoulders, chest/ribcage, skull). When Qi enters and feels full in the area your training, on inhale, draw in new qi. On exhale, spiral existing qi in your body around the bones and imagine it compressing deep into your bone marrow one body part at a time. After a while, it should yield a feeling as if your bones are iron wrapped in cotton. Then once you can do the whole body, try doing it simultaiously. Then you can lead your qi strongly anywhere in your body, and use bone breating to practice your Taichi using this method. it will feel like your a floating skeleton. Then when you advance and pack enough qi in your bone marrow, it will yield a vibrating sensation which will get stronger the more you practice.
The second way is compressing breathing. imagine you are just a skeleton, and imagine your chi compressing into your bone marrow one section at a time (see above). Once you can feel your qi compress to your bone marrow everywhere, practice the whole body simultaniously. Then when comfortable doing that, compress deeper, and eventually the qi will begin to vibrate. compress deeper and deeper until the vibrations are substantial. They may also manifest externally. Then you just try to control the vibrations, making them faster and stronger, and slower and weeker till you can "regulate without regulating".
The compressed vibrating qi is considered the chi used in jin by some tai chi masters. Some dont teach anything like that. two references is Mantak Chias Bone Marrow Nei Kung, and The Tai Chi Classics by Waysun Liao.