Just as a bit of an update. A few people have asked me exactly what is happening in the video.
Basically:
1) Cast ANY spell (Lightning Bolt, Multi-shot, Apollo's Shield; whatever).
2) Either watch the middle of your screen or watch your toon
3) Once you see either the name of the spell appear in the middle of your screen; or see your spell ability icon go away over your head, cast your transformation button. The shortcut key for this is your spacebar.
4) Watch your toon
5) Once you see your sylph transformation icon go away hit your Rune button. Hitting this before this point will cause you to not pop the rune.
6) You will transform into your Sylph and your sylph will immediately cast your rune that you qued up.
It does take a bit of practice but I have been able to do this trick in both Battlegrounds, Arenas, as well as the World Boss for both Brutal Rune as well as my Blood Rune.
Finally there has been some concern that this is some type of exploit to the game. A moderator on the Wartune forums has specifically posted about this saying that it is an intended function of the game:
http://forum.r2games.com/showthread.php?101234-How-to-cast-runes-in-Sylph-form.&p=808691&viewfull=1#post808691