Enable pin connects or disconnects the motor to the H-bridge. When connected, the motor will spin clockwise or counterclockwise or will be braked. When disconnected, the motor can freely spin. Also the Enable pin may be used for PWM.
00 or 11 connects both ends of a coil to the same potential, eighter GND or Vcc. It does not matter which one of them are you using, they both do the same thing: shorting the coil. So, when the motor is spinning and you short the coils, the motor becames a generator and the voltage generated will go back to the motor and tend to generate a rotation in the motor but in the opposite way the motor is spinning, thus rapidly slowing it down. Never used this for steppers. To slow down a stepper, just give it steps at a slower rate until it stops. Then, if you keep the coils energised (01 or 10), the stepper will hold it's position. Depending on the force needed to hold that position, you can short both coils, energise one coil and short the other, or energize both coils.