A chakra is an etheric organ.
The moment you concentrate on it, you feed it with etheric energy.
Your etheric body is your energy body and the moment you focus on any specific area of your being, you bring an extra flow of etheric substance to that area.
Imagine a muscle. When you want to build it up, you train it. After a few weeks you see an increase in its size and power.
Chakras function in a similar way.
You can build them up by “feeding” them with energy.
Concentrating on them is one of the most direct ways to bring an extra flow of etheric substance into them.