In doing internal power training (like 24 style tai-chi Neigong), one needs to train both overt and covert power. By covert power, the power source is from one's extremity mainly referring to one's stretched hands and fingers. With one's stretched fingers energized, Qi will be made to flow via one's arms to one's shoulder blades and one's torso. In application, power will be generated from activation of one's energy source from one's torso (a bigger mass than one's fist), as triggered by one's stretched hands/fingers. Without such transmission, it is not covert power, just brute force (which, by definition, uses only the power of the fist and forearm). For advanced practitioner, the power triggering point will be one's middle finger. A closer look at the statue of Bruce Lee's combat stance in HK's water front will let your understand what I am talking about.
In some expositions in internal martial art texts, overt power has sometimes been downgraded. It is actually very useful. The use of overt power is more versatile and higher speed can be achieved without too much loss of power for the trained practitioner. In certain situation, full-body power cannot be generated, like when one is lying on the floor on one's back. A prominent MMA fighter comes to my mind, the pound-for-pound champ Andersen Silver. With his back on the ground, Silver can deliver TKO punches or powerful punches that stunned his opponent so that he can execute a successful submission arm lock. Silver can be seen doing speedy Wing Chun wooden dummy work as an expression of overt power (see below).
Covert power is actually Wang Xiangzhai's "muscles-as-one" 肌肉如一, and it is the definitive training objective of all internal martial arts. Training-wise, it demands more internal relaxation/loosening (song 松) in the training process. In terms of stages, it is best to start with covert training, and the best training concept in overt training is Wang Xiangzhai's "point-stretched, body-relax/loosen" （點緊身松).
Needless to say, a single post cannot go into details the relevant training methods. Interested readers can search for relevant posts in this blog.
Anderson Silver doing Wing Chun wooden dummy