"You should read the documentation and learn WP before making untrue statements like this."
I have. And you shouldn't assume that I haven't. Perhaps you misunderstood me? Let me give you an example of how I understand it:
1. You create a page template called "parentpage.php", which WP recognises as a template because you have
"<?php /* template name: parentpage */ ?>" in the header.
2. You create another page template called "childpage.php", which WP recognises as a template because you have "<?php /* template name: childpage */ ?>"` in the header.
The two template files are only different in that "parentpage.php" has a sidebar and "childpage.php" does not.
Every time you create a top-level page (no parent), you specify "parentpage" as the template. Every time you create a sub-page (parent exists), you specify "childpage" as the template.
That's one way to get no sidebar on secondary pages. Another way is to just modify the existing "page.php" (if your theme has it) as I already suggested.
If you read the original post again, I think you'll find what I suggested is actually less error prone because the template itself determines if it's going to show the sidebar. The author doesn't have to remember to select a "childpage" template. The downside to my approach is that you can't specify that you want a sidebar on some of your secondary (sub) pages.