Complex sentence: Pop-under ads do not immediately impede a user's ability to view the site content, and thus usually remain unnoticed until the main browser window is closed or minimized, leaving the user's attention free for the advertisement.
Split into simpler sentences: Pop-under ads do not immediately impede a user's ability to view the site content. They usually remain unnoticed until the main browser window is closed or minimized, leaving the user's attention free for the advertisement.