Understanding "Send me email notifications even when I am active on the site"

[axp (Alexander)]

Can somebody please explain me how this setting in users pref works?

I can’t translate this correctly because I don’t understand how it works and I can’t repro this here on meta. Thanks!

[codinghorror (Jeff Atwood)]

We do not email you by default if your browser is actively pinging the site, because we would be double notifying you – once in the browser and again via email.