[General] Commas

It is the fruit trucks that are most rigorously checked. They typically carry crates of watermelons.

The commas in the original are fine, in my opinion.

I don't understand it.

Piled on top of each other(, end of subordinate clause, beginning of main clause) they are (, beginning of parenthetical note) the authorities say(, end of parenthetical note, return to main clause) a classic place for immigrants to hide.

They sit(, end of main clause, beginning of subordinate clause) heads bowed.

Thank you
Piled on top of each other, they are, the authorities say, a classic place for immigrants to hide.
I don't know very much about main clauses and subordinate clauses.
I think a subordinate clause can't stand alone.
The words or rather the clause 'the authorities say' is not essential to the sentence.

So the following is fine.
Piled on top of each other, they are a classic place for immigrants to hide.
I think I am correct.

Tina3 said:

Piled on top of each other, they are a classic place for immigrants to hide.
I think I am correct.

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***** NOT A TEACHER *****

I agree that you are correct.

The words "the authorities say" are an extra remark that the author "threw" into the sentence. You are 100% correct:

it is not essential. That is why, as 5jj said, it is called a parenthetical expression. The writer of that sentence wanted

to make it clear that it was not he/she who was making that claim about hiding immigrants. It was the authorities. So

if the information in the sentence angers a reader, the writer can reply: Don't get angry with me. Direct your anger at

the authorities.

Maybe if we punctuate it in another way, this idea would come out more clearly for you:

Piled on top of each other, they are (the authorities say) a classic place for immigrants to hide.
Piled on top of each other, they are -- the authorities say -- a classic place for immigrants to hide.

*****

"They sit, heads bowed" seems to interest you. Would rearranging the words help make it clearer for you?

Heads bowed, they sit. = With their heads bowed, they sit.

They sit, [with their] heads bowed.

Thanks for the replies.
They sit, heads bowed.
If you remove the comma, does it change the meaning?
Such a short sentence doesn't need a comma; that is how I feel.

Tina3 said:

Such a short sentence doesn't need a comma; that is how I feel.

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You raised the question and I, a teacher and native speaker, gave my opinion. If you prefer to follow your own feelings, fine.

TheParser said:

But I think that ordinary people usually follow the rules. I am 99.99% sure that 99.99% of English teachers would give you a lower mark if you left out the comma.

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I would not give a lower mark, but I would certainly point out that a comma is needed there.

Thanks
You have written the following:
It tells the reader to pause.

I have heard this before.
So you can place everywhere to pause.