setPoweris in a range of -1 to 1. The absolute value of the assigned number determines percentage of duty cycle. As an example, 0.3 and -0.3 both indicate that the motor is operating at a duty cycle of 30%. The sign of the number indicates the direction the motor is rotating in. To better understand, consider the following graphic.
setPowervalue between -1 and 0, the motor will rotate in the direction it considers to be reverse. When a motor is assigned a value between 0 and 1, it will rotate forward.
setPowervalue for the right motor is less than that of the left motor. This is exhibited in the following example.