You clearly know not the science of what you speak. The law of specificity should be considered with every drill, every exercise you ever do. What specifically does punching with a dumbbell train? It trains you to get good at punching with a dumbbell, not at punching without one. To punch better, practice punching. If you want to be faster, train the nervous system to be more efficient. If you want to be more powerful in your punch, train for strength.
Here is an example of one way to greatly increase your power in a punch.
1. Get your close grip, three board bench as heavy as humanly possible, keeping your elbows tucked as much as possible the whole time.
2. Mimic your punch with exercise. Lay on an incline bench with a dumbbell in hand with extended arm and hand in prone position (end of a punch), then lower the dumbbell to the position you would start your punch from. For me my elbow is tucked, hand neutral and close to my cheek. Basically punch with the dumbbell straight up. You will use a weight that allows you move down then up again in just less then 1 second, if you are slower, it is too heavy. Do two reps per set and no more then about 8 sets. This is stressful on the elbow, if you experience elbow pain, go back to 1. or decrease your range of motion.
3. Punch a bag, do this with great power on every punch, if you lose punching power you did too much. Next time stop before you lose power.
4. Punch the air. Do this fast like lightning and with perfect form. When your form breaks down, you did too much. Next time do less.
5. Use these same principles with the reverse of your punch. You figure that one out, or this will turn into a full blown article.
- Trains max strength, 2. Trains motor recruitment, 3. Trains power (puts 1 and 2 together), and 4. Trains coordination and speed. 5. Trains balance of strengths which makes you faster then just training the puch. (your body will only allow you to push with as much speed and force as your body can pull back with speed and force)
1 + 2 + 3 + 4 + 5 = holly shit punch.