Which technique provides a safety backup with minimal extra friction?

The technique that provides a safety backup with minimal extra friction is the use of an ATC (Air Traffic Controller) with a prusik safety. This method is effective because the prusik knot is tied on a separate, static line that holds the climber securely in place without significantly increasing the friction in the primary belay system.

In a rescue context, using a prusik as a backup enables a rescuer to maintain a smooth and controlled descent or ascent while ensuring that if the primary system fails, the prusik will automatically engage and hold the weight. This is crucial in rescue operations where minimizing the risk of failure is paramount, and any added friction could complicate the movement or lead to unintended slack in the system.

Other techniques may provide safety in different ways but might introduce significant friction or complications that affect the efficiency of the system. For example, the parallel plaquette technique might surface some added friction due to its setup, and a Munter hitch redirect is less stable as a backup since it can add unpredictability in the system under load. Meanwhile, coils in the pack are not a safety technique but rather a method of carrying rope, which does not provide any backup scenario for the climber or rescuer.