The secret is a good core training
I think everyone here agree with the fact that you hit harder when you use your entire body, when you use this technique to punch, you're using your abs, your back, your obliques and your legs. Your arms just need to be able to resist to the impact that the punch will make, I suggest plyometrics (Medicine ball). Kettlebell exercises are really good too.
and add to this a lot of practice to be very familiar with the movement, so your punches could become really fluid and be relaxed until you hit, because tension in your muscles would retain your power.
Did I forget something?