Shotokan punching is often taught from the standing posture in extended long posture called Zen Kutsu Dachi. Every so often carried out by marching up and down switching between left or right leg forward,long postures or other occasions on a Thai pad held by a peer, the power is created by pushing the hip forward.
The difficulty with this punching protocol is that the rear leg is so extended that it behaves as an 'anchor', pulling on the hip in the opposite direction to the punch. This is exasperated if the student is encouraged, like I used to be, to keep the rear heel flat. To summarise, the straight leg and foot held down pulls back on the forward motion of the hip, detracting from the power of the punch.
In the same way that an anchor trailing behind a ship leaving port is suboptimal, so is the traditional karate method of throwing a punch. The following three tips will drastically improve on the traditional karate punching method.
ONE - Ground Reaction Force
By using the feet to press into the floor to start walking 'ground reaction force' pushes back as the ground is solid (or nearly). This force starts the forward movement. This ground reaction force is present in all manner of human movements and so it is important we emphasise it in karate punching! Therefore, when punching from zenkutsu dachi it is necessary to have a bend at the knee and a raised heel to maximise the forward movement when the foot is pressed into the floor. Of course, as the heel should be on the floor in Zenkutsu dachi the heel can be raised minimally.
TWO - Momentum between stances
In sport karate when delivering a reverse punch the foot will roll onto the ball with the heel raised. This detracts from the anchor effect by allowing the hip greater freedom to move forward. This effect can be increased by pushing the foot forward after it presses into the floor, which allows momentum to pass from the foot to the hip and onto the fist eventually. Momentum moving toward the target will greatly improve power generation in the punch.
The equivalent of this in traditional karate is for the punch to involve a forward action from zen kutsu dachi to han zen kutsu dachi, long to half long stance. This allows great forward movement at the hip, a good thing.
THREE - Whipping effect
In boxing, many big punchers build a whipping effect in their strikes. We can consider this as the sequential punch. There is a specific organization to the way through which a punch is delivered. The foot pushes into the ground, driving the hip forward before the shoulder moves. The shoulder is 'left behind and is driven forward by the stretch created between the hip and shoulder. This then tosses the fist on the way to the target. The opposing actions of each body segment rather than acting like an anchor, work in harmony to 'sling' the following segment toward the target. It's similar to the final part of a discus throw, where the hip pulls the shoulder, which pulls the arm and fist to the target. This reciprocal action creates hugely powerful force.
I can guarantee that your karate punches will produce much more power if these tips are incorporated into your technique. A fair amount of effort is required to differentiate the hip and shoulder actions but it is well worthwhile for the power improvements. The punching action of most karate is missing these attributes, although occasionally they are present.