ThinkorSwim just released a 7 minute video tutorial on Portfolio Margin. I've seen a few video overviews of PM and this one is probably the best I've seen so far. It's short, but the video presents the main concepts of Portfolio Margin and is reasonably accurate in its content. Kudos to TOS for their effort.
Jason Apolee is a contributing editor to The Margin Investor where he focuses on news commentary and evaluating broker offerings.