Distinguish between weight and pressure:
In all stances (except one leg stance) the weight is in the center, and the pressure is to one leg or another, (not as some books say that 70% of the weight is on one leg and 30% the other) depending on the stance and technique direction.
This allows for interaction between both legs and the body center, to create potential energy or for kime.
In front stance the pressure (generally) is toward the front foot, using the breath to create pressure from back foot through body center toward front foot.
In back stance use the breath to create pressure from front foot through body center to back foot.
Stance form is meaningless without the right interaction of both feet, body center and breath.
Take back stance for example, the form, position of back stance is useless unless there is interaction between both legs, and the front foot make pressure to back leg, to create coiled spring in the stance.
Pressure is applied differently depending on necessity.
This pressure is applied differently at the moment of kime than when moving around with opponent. In case of kime it is maximum pressure and in the kamae it is soft pressure to keep potential energy.
Either foot can receive pressure at any moment so one can move at any instant to any necessary direction.
Wheel of energy.
The stronger the pressure applied to front foot in front stance, the stronger the reaction to back leg and more potential energy.
The same is true to back stance or any stance.
This principle is true at the moment of kime or in Kamae stance when moving with opponent.
So at each kime there isreaction to back leg which means there is potential energy and no gap between techniques.
Front stance is the only stance in karate that the back leg, supporting leg, is pushing and directed to technique direction.
All other stances are indirect, in back stance one presses back and twist the muscles around the back leg thighbone back to create energy forward, so is sochin or kiba dachi.