When the two uncharged and disconnected capacitors are connected to their respective power supplies, the capacitors are charged to the voltage of the power supply. When the capacitors are fully charged, they are disconnected from their respective power supplies, and one of the terminals of a capacitor is connected to the other terminal of the other capacitor (the switch between the two fully charged capacitors is closed). When the capacitors are connected in such a way, the charges will flow for a short time until both capacitors have the same potential difference across them.
