The heart of the "Neijia" or the "internal" martial arts is in the way the body is utilised. All of the Neijia arts have exercises outside of martial application mode with which to practice this form of movement. All of the "forms" (Xingyi forms, Tai-Chi forms, Bagua forms, etc.) 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 emphasise the style of movement fundamental to the Neijia: the Yang style Tai-Chi 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; let's focus on some suggested criteria to keep in mind while practicing a set of reeling silk exercises (Chan Si Gong). These suggestions are meant for people who already do reeling silk 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 with. 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 "Dantian" area, it will get a more intensive workout as the external motions get smaller.
4. Do as many of the exercises as possible with 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 "transmitters" 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 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 opening), 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").
Mike Sigman has written an comprehensive account on the requirements for achieving this baseline skill of Taijiquan and the neijia: Silk Reeling, aka Six Harmonies Movement