The deeper inside that your Qi is sunk, the more advanced and smaller so too will your Tai Chi form practice. There are certain points on the body that are responsible for this sinking of the Qi into the Tantien but these are not apparent until your form practice has come to a high level.
The so-called "Third Eye" point or "Upper Tantien" is one such point that blasts the Qi right down into the tantien in conjunction with the Girdle Meridian or Dai Mai. It is the Dai Mai that is the "fan" that explodes the Qi out to all parts of the body especially in moves such as Brush Knee Twist Step. That Qi then accumulates at the Third Eye Point where it will be felt for a moment as a sensation. Everyone feels something different so it is useless to try to explain my own "feeling " at this point. The Qi, is then, along with the next empty part of whatever posture you are making, pushed rapidly right down into the tantien. This is turn will cause your form practice to become very tight, small frame and slow, so much so that you are unable to even do most of the explosive moves physically.
The Qi is sent down through the Tantien from the upper Tantien via the "Jung mei" or Life Force meridian. It travels through the middle Tantien and in so doing aligns the three Tantiens. This will align the backbone perfectly so that each vertebra is "connected" electrically to act as a small capacitor storing Qi or electricity. When all of the vertebra are connected in this way, we are able to deliver great power from the backbone.
Many think that this alignment is a physical thing that happens but it isn't. As the Qi passes through the Jung Mei to the lower Tantien, it simply gives us a thought of where the correct alignment should be for the greatest power and flow of Qi. We then align the backbone so that we feel the least tension and resistance as this is where we feel the best. Any tension of external though process at this point will stop this process from happening though and this is why it is often difficult to get to this level of training. The physical moves with the mental (internal) and steps us up the ladder. But you must have the physical correct first. Once this physical is correct, the mind is the next part. And although it sounds simple, it is not! You must stop all conscious thought. This is the only way to get to a higher and deeper level of advanced Tai Chi training.
This in turn will cause your own form to advance and once in this "zone", you just have to slow down to a crawl, taking at least one hour to perform the whole set of movements, often much more. For instance, my morning session took me 1 hour and 20 minutes and I didn't feel a thing until after the training and my legs told the story as well as my upper body from holding my arms up for that time period. Only when doing the form this slowly is one able to feel (but not think upon) each energy flow for each movement. And often these energy movements are quite complex. Some will flash across your eyes, while others will combine with other meridians and move right down to the ground via the feet. Others will happen only in the palms of your hands in the form of very quick and tight circles making your hands feel like iron. And this is where the say of "Externally like cotton, internally like steel" comes from. In fact you cannot place any of the old classic sayings by the old masters over a beginner's or even intermediate level form as the old masters only wrote the classics, when they were indeed masters and with reference to their very advanced tai chi form practice.
I always tell my students, "Just Do It". In other words just DO your tai chi form, do not try to do very low movements in order to look good, do not try to do martial movements, just DO it, just step and make hand movements. It really is that simple!