In these videos, Joe McNally walks through the basics of TTL flash, including lighting ratios and high-speed sync. He’s specifically talking about Profoto’s Air TTL system; however, these principles apply to smaller speedlights from Canon, Nikon and other manufacturers.
If you like these videos, I highly recommend checking out Joe McNally’s book, The Hot Shoe Diaries, which focuses on using TTL speedlights in a variety of situations.