The heart of the "Neijia" or the "internal" martial arts is in the way the body is utilized. All of the Neijia have exercises outside of a martial-application mode with which to practice this kind of movement. All of the "forms" (Xingyi, Taiji, Bagua) are really a series of linked martial applications (or they were originally) which use the "internal" form of movement... however, the movement of the internal styles can be done without having to do a martial-based form.
There are various qigongs which emphasize the style of movement fundamental to the neijia: the Yang-style taiji qigong, the Xingyi neigongs, the Chen style Reeling Silk qigongs, etc. All of these qigongs and all of the internal styles are quite capable of producing "internal strength".
Because of the sophistication of the Chen style reeling silk exercises and because they are becoming popularly known, though, let's focus on some suggested criteria to keep in mind while practicing a set of reeling silk exercises. These suggestions are meant for people who already do silk reeling exercises; you cannot learn to do them from words.
1. Concentrate on keeping a complete and relaxed peng path at all times "to" the area which is being moved and move the area by "pushing" it with peng and "pulling" it with contraction/closing/down-weighting vectors.
2. Always pretend that there is someone holding their hand on the part being moved; someone who is constantly monitoring the completeness of the ground path (peng) and the relaxed nature of the transmitted power.
3. Use large movements with the whole body to start. Gradually, over a period of months, make the movements (envisioning the same "monitoring") smaller and smaller. Because the peng path goes through and is manipulated by the "Dan Tien" area, it will get a more intensive workout as the external motions get smaller.
4. Do as many of the exercises as possiblewith the knees bent somewhat (do not go so low that the thigh goes below a 45 degree angle, though). This will greatly strengthen the leg and hip muscles.
5. Do the exercises from the head down to the toes.
6. One of the main goals is to train peng power, down (closing) power, and body inertia to the arms... so that the arms are "trasmitters" of this power. Keeping "very" relaxed and concentrating on keeping these powers in the arms will bring them there. Local strength in the arms and shoulders will only deny progress.
7. There are really 6 directions of power: push out (peng), pull in (peng from behind and contraction), lift up (peng vector), weight down (closing and weight vector), waist-turn forward (peng combined with closing) and waist-turn backward (peng combined with closing). All movements are variants of these 6 directions. These 6 directions are really only variants of peng and closing.
8. In all movements downward or backward/downward, there should be a very slight storage in the lower back.
9. The exhaled breath goes with the imagined application of power, the inhale goes with the storage.
10. The peng should be felt at all times in the cycle of a movement (no gaps, no deficiencies).
All material is reprinted here with the permission of Mike Sigman.