The RED's cams have a positively locking spring, which means that the device is self-parking rather than self-tailing. This is important for Rope Access work techniques as they require the back-up device to be positioned so as to reduce fall factors to a minimum. The RED makes use of 'Rotational Braking' which means the device rotates clockwise creating friction and resistance, which then results in very effective automatic braking in the event of a sudden pull on the cows-tail such as might occur during a main line failure. 
 
Other Versions:
To increase the effectiveness of the Rotational Brake in a panic situation (for example, a main-line failure), the RED can be fitted with a 'Popper' tow cord release clip.  For further information on the Popper clip version, see RP892.  ISC also manufacture a No Cord version, see RP890