Basically the iPhone App connects to the Bean+ board trough BLE (Bluetooth Low Energy) and send serial commands to control the speed and direction of the motors.
The TB6612FNG is responsible to send power and the polarity for each motor.
For the sake of simplicity, in this project I have used two different power sources. The Bean+ native one (3.7V battery) and a external 5V power supply for the motors. A single source can be used as long as it respects the maximum voltage of each component.
You need to power the TB6612FNG with 5V from Bean+ on the VCC/GND pins and supply motor power to the board on VM pin. It is important to note that not all Bean+ pins have PWM support. According to the specs, only the D2, D5, D6, or D7 pins have it. The iPhone App communicates with the board through the Bean-iOS-OSX-SDK