Why do electricians work with one hand behind their back?

It's not a universal rule or anything, but some electricians only need 1 hand for reconnecting wiring or what-not. So keeping the other hand behind your back keeps you from placing it on static sensitive equipment or electrical circuits.

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The reason is much more substantive. It is this:

You don't get shocked by touching an electrically-live point, no matter how high

its voltage may be. Birds sit on bare multi-kilovolt lines every day without harm.

You're harmed only if current passes through part of your body. In order for that to happen,

you must be touching two places that have a voltage difference between them.

So the best way to avoid most dangerous situations is: Wear shoes with thick

rubber soles, and keep one hand away from anything that conducts electricity.

This makes it pretty difficult to get into a situation where current can flow into

you at one place and out of you at another place.

The experienced electrician is constantly aware that he has two big efficient

electrodes hanging from his shoulders. To avoid setting up a conducting path

through his body, he keeps one electrode behind his back, or in his pocket.