With the release of WordPress 3.1 came a new version of the stock TwentyTen theme with some subtle changes, most importantly the move of all the loop elements in the templates (the code that runs through the database and gathers info like title, date, author, categories, content, images etc) into their own files. In the first release of TwentyTen this was restricted to just the single post view (loop in loop-single.php) but now it also extends to among other files the page tempalte (loop-page.php).
The problem arises when you want to create custom page templates. Previously all you needed to do was copy the page.php file and make the changes you wanted in the loop. But with the new system copying the page.php file alone won’t let you make any changes to the loop, just the overall structure (header, sidebar, footer etc). To make changes to the loop you have to either copy the loop code out of loop-page.php and into your custom page template or use a conditional statement along with the is_page_template() function in the loop-page.php file to detect that a custom page template is being used and then make the changes. The video above demonstrates both approaches.