The Enable pins (1 and 9, 16 is motor power...) let the outputs receive power or not. If you PWM them the motors will be ON and Coasting (off, but no power). If you tie the Enable pins to Vcc and PWM one of the direction pins, the motors will be On and Braked (both motor leads are shorted to ground). There is another method, Enable tied to Vcc and PWM both direction pins (eighter using complementary PWM outputs on the microcontroller, or using an inverter between the direction pins). This last method is the best because when the motor can recharge the batteries when the generated voltage is greater than the battery voltage (plus the protection diode).
So, for a good engineered drive system you should use DC motors with H bridges connected in the regenerative mode (the last method for PWM).