Can anybody explain this with example?

I read this on the internet : Never use "will" in the clause where you use "while/when/before/after/by the time/as soon as/if/unless".
For example :
I will call you when I will arrive. (Incorrect)
I will call you when I arrive. (Correct)

I am a little confused about its usage. Does it mean wherever we use "while/when/before.....", we shouldn't use the future tense in that clause? What is the actual meaning of time clause here?

Thanks.

Dear meg,
The rule is very simple.
First let me come to the meaning of WILL. It means an affirmation to do something (or to not do something). When I say "I WILL call you", it means that I have promised to do something (in this case, "call you.)
Now coming to the use of WILL while using "while/when/before/after/by the time/as soon as/if/unless". If you will note, all these are future tenses. All these are yet to happen. When I say "I will arrive" it means I am certain of my arrival. How can anybody be certain of the future. I may never arrive. This is the reason why the use of WILL (which is confirmatory in nature) is prohibited with things/events to happen (the future is uncertain).
I hope this helps you.
If it is not clear, I WILL do it when I WILL write the next time. (Got the message--I may not write at all.) So, the correct usage is "If it is not clear, I WILL do it when I write the next time.

Dear all,
Thank you so much for your help. Now I got it correctly.
Regards.

Thank God! I didn't feel faint while reading that much. Hey Jim! Thanks a millon! How kind of you writing all this detail!
Anyway, I am planning to start to take English lessons from you. Where do you live? You really have extensive knowledge of English. My hat off to you!
Regards.

Thank you, CalifJim, for your very clear and very useful explanations.

1. I agree with you, and I don't like the oversimplified, formalistic way of expressing grammar rules, as: "Never use... after...": as you said, "never" is a strong word!
Teachers sometimes seem to think that, if they explain things a little better, their pupils will be confused; but the truth is that pupils think (some of them at least), and they'll be disappointed when they see (here's our rule!) that the pseudo-rules they've learned are hardly ever true: which is not the fault of the rules themselves, but of the unaccurate and false way of expressing them. Of course, we mustn't be unnecessarily complicated, but I think that it's important to be accurate.

2. As for this rule, perhaps I would express it so: "Whenever, in a sentence, the main clause contains a future tense, the dependent (subordinate) clause cannot contain a future, too: its verb must be a present".
What do you think of this formulation? I think that it might be accurate enough for practical, didactic purposes.

3. Here's an example of how I'd explain it:
"When I get back home, I'll read your letter."
Here we have a sentence, with a main clause: "I'll read your letter", and a dependent clause: "When..."; since the future ("I'll...") is used in the main clause, it can't be used in the dependent clause: "When I shall (I will, I'll) get back home, ..." would be wrong; we must use the present instead: "When I get..."

4. This rule is particularly important for those students of English whose mother tongue is one that would use two future tenses in such sentences: both in the main and in the dependent clause. That would be the rule, for instance, in Italian.
I would then explain the thing with a comparative (contrastive) method: I would ask my pupils: "How would you say that in Italian (French, Spanish, etc.)?", and I'd point out the difference: two future tenses in their mother tongue, only one (in the main clause), and a present (in the dependent clause), in English.

5. If we express the rule like that, we needn't enumerate the conjunctions after which, or the clauses in which, the future tense mustn't be used.
These are mostly temporal or conditional clauses; but I think that it would be wrong to say, e. g., *"The first who will get back home, will read the letter": it should be "...who gets"; and this is a relative clause.

6. What do you think of this? English isn't my mother tongue, and I'm only trying to learn it.
And now, on second thoughts, I'm not sure that my formulation was right: perhaps it's too general; for example, would this sentence be wrong: "Next year I'll be very sad, because you won't be with me any more"? Should it be "...because you aren't..."? I don't think so. So, perhaps IT IS necessary to enumerate the clauses in which the future would be uncorrect.

7. Just one more question. Please, bear with me (my heart is in the coffin).
Both "shall" and "will" can be used as auxiliary verbs to form the future tense, but they can also mean "must" and "want" respectively. So, would it be possible to use them in a dependent, e. g., if- or when- clause (with a future in the main clause), with these second meanings? I think I've found some such sentences, but I'm not sure, and I can't think of an example now: could you give me one (or two)?

Thanks for your patience, and for your precious help.