Rule 9 of 40 · Chapter II — The Body and the Current
Keep one hand behind your back
Why this rule exists
The deadliest shock path runs hand to hand, straight across the chest and through the heart. Put both hands on energized equipment and you invite exactly that path. Work one-handed, the other hand in your pocket or behind your back, and a shock is far more likely to travel hand-to-same-side rather than across your torso. It is a small habit that changes where fault current goes. The one-hand rule does not prevent shock; it biases the path away from your heart, which is the difference that saves lives.
In practice
When you must be near energized parts, work with one hand and keep the other out of contact entirely, pocketed, behind your back, or at your side away from any grounded surface. Stand on a dry, insulating surface so your feet do not complete a circuit to ground. Do not lean your forearm, elbow, or shoulder against a grounded enclosure while your working hand is near a conductor. Better still, de-energize first so the one-hand rule is a backup, not your only defense.
When it doesn't apply
The one-hand rule is a mitigation for unavoidable proximity to live parts, not a license to work hot. If you find yourself relying on it as your primary protection, stop and ask why the circuit is not dead. It also does nothing against arc flash, which threatens you without any contact at all. De-energize whenever you can.