Greetings..
Art is correct in that without contact we have no feedback as to what actually occurs when the punch is delivered.. having trained in several external styles and finally focusing on the internal, i notice a big difference in punches.. much of the external is concerned with arm power and secondly twisting power (waist/legs).. Taiji also focuses on waist/legs but also uses simple kinetic energy of the whole body moving forward as the weight is shifted into the punch.. the shifting of the weight from a rear weighted stance to a front weighted stance is loaded with kinetic energy (like putting your feet under the heavy bag then pushing it away and letting it hit you full-force).. then, as your weight shifts forward, you also twist the waist accelerating the striking hand.. just as the punch nears its target, the front leg brakes the (let's assume a left foot forward stance) left side of the pelvis while the rear (right) leg pushes the right side of the pelvis forward, rotating the spine and shoulders in a whipping action.. the arm is relaxed but with substance and is whipped toward the target.. here, correct timing is crucial, the frame (skeleton) very briefly locks into place to add support to the impact of the punch.. then, just as quickly, the frame relaxes and prepares for the next technique.. this only addresses the physical movement of the punch, involving Qi is a seperate issue, too long for this post..
Practice on a heavy bag is very useful in adjusting timing and frame for maximum effect.. ultimately, you should see the heavy bag (Muay Thai type, long and sort of narrow) bulge substantially from the whipping impact but not much overall movement.. big overall movement, like huge swings of the bag are commonly associated with external energies.. while huge bulges, almost folding the heavy bag with little swing, are commonly associated with internal energies..... or, at least, that's how i have been taught.. and it works well for me..
Be well..
TaiChiBob.. "the teacher that is not also a student is neither"