Electricity forces your muscles to contract, Which means whatever you are holding you cannot let it go whilst it is running through you still. It's involuntary.

your muscles are contracting depending on how hard you’re being shocked, so you cant let go

Muscles are told to fire by the electrical impulse that comes from the body's own nervous system.

The electricity coming from the electrocution source is more powerful than the body that any impulse to open is not received.

The hand in particular has a design which favors the closing motion and not the motion to open. Both sets of muscles are stimulated at the same time by the electrocution source but the closing muscles hold the object in the hand.

If the current is DC, the current causes all muscles to contract. This closes your hand and prevents you from letting go.

If the current is AC, the muscles spasm, and you probably will let go.

In power stations and such, high-voltage direct current is common, and a short can electrify almost any piece of metal. To protect themselves workers brush the back of their hands against metal objects before using them. This way, if the muscles contract, they will violently remove your hand from the object, not grab it.

The signal from your brain that tells your muscles to squeeze is electricity. When you squeeze all the muscles in your arm, your hand closes tightly.

So when you're electrocuted, all your muscles are just getting the SQUEEZE signal from all that electricity, louder than your brain has ever sent it before. So your muscles squeeze like crazy, locking your hand closed.

And if you try to let go, your brain's own weak electric signal saying "let go" is WAY weaker than the electric jolt yelling SQUEEZE. So you squeeze.

Same reason why some police use tasers, as it stops an attack immediately.

That’s only half right. All muscles ARE controlled via electric impulse from the brain/nervous system, but it isn’t the “squeeze” command being given. ALL affected muscles are told to contract. Your hand has much stronger muscles for closing/gripping than for opening. (It has to overcome much greater force to hold things than to release them, so this makes perfect sense.)

When I was a kid we were told that if we needed to know if the electric fence was on to touch it with the back of a hand so your arm would pull away if it was live.

How does touching the back of their hand to metal protect them?

The theory is that the muscle contraction pulls your hand / arm away from the electrified thing you touched, rather than pushing you on to it.

Well the first step is to verify power is off, then check with a meter or detector, then the final step is to use the back of the hand.

They are usually too shocked to do anything.

Esentially all signals in your brain and muscles are due to electricity. This much triggers them all and causes them to contract.

Instructions unclear, I backhanded the meter.

Normally your brain and nervous-system activate your muscles.

External sources of electricity also will activate your muscles, even if your brain doesn't want to.

Generally the electricity tends to 'tell' all your muscles to tense up.

Your 'close hand' muscles are stronger than your 'open hand' muscles, so they'll win this fight and 'succeed' in closing your hand.

Your brain has no say, really, because any orders it gives are overpowered by the external source of electricity.

This is why it's so easy to hold a crocodile's mouth closed!

I think you have that backwards. AC causes muscle contractions. If DC did, you wouldn't see videos of people locked on to things that have become electrified, because the power from the plant is AC. That's why Edison electrocuted an elephant.

If you touch with the back of your hand, you might get shocked but when/if your hand locks up it won't grab on to anything. The electricity itself isn't holding you, it just interferes with the way your brain talks to your muscles and you end up holding yourself.

Nope.

AC can induce muscle tetanus, but while it can cause you to lock a hand to something, DC is quite a bit more likely to do so. AC has a greater chance of causing a spasm that either removes the hand or allows the victim to pull free.

AC, however, is more likely to kill you, as it can trigger ventricular fibrillation. DC is more likely to stop your heart, which can actually be easier to recover from.

As for Edison, he electrocuted the elephant and invented the electric chair to illustrate the dangers of AC current. Not because he thought it would be all that dangerous at household voltages, but because he owned the patents on DC and wanted AC to look bad. Look into Edison a bit, you will see that he was something of a jerk.

However, for local grid purposes, AC is much better and won out in the end.

The electrocution itself causes the muscles to tense and grip. Normally when you use your muscles, nerves send electrical signals as action potentials.