Thanks for the comment, Steve!

Greg, the video concentrated on the mainsheet to shape the leech with the boom still over the boat (i.e. close hauled). So, the vang will not have a lot of effect until the boom moves over the water. So, for example, if you have too much twist in the sail, you can add a bit of mainsheet tension to add fullness aft, which will move the draft aft. You can fine tune draft location with the halyard or Cunningham. You might also check out my earlier video "Mainsail Draft Secrets for Sailing".

Thanks, Dougyelnats!

The second illustration shows a sail that's way too flat in the back half of the mainsail. Follow the curve of the sail all the way across from the mast to the edge of the leech. Note how it forms somewhat of an elongated "S" shape. On your mainsail, the most power of the sail comes from the aft section. If you over flatten this section, you lose power. Compare this to pressing down on your car's  accelerator; then taking your foot off the accelerator. The car keeps going forward, but loses power.  This sail will still drive the boat forward, but with a big loss of power.

There will be times when you want to "over-twist" the leech to dump air off the top of the sail. In high, gusty winds, you may want to depower the mainsail to keep the boat on her feet, and weather helm and heel under control.  Of course, you will want to reef as soon as possible, but depowering techniques are good to know. But this video was not about heavy weather techniques, but basic sail trim in moderate sailing conditions. Also see my video on "Mainsail Draft Secrets for Sailing".