This is quite a simple trick, it is done during the bonus objective "The Fallen Tree" in Ardenweald. There are captured Sylvar that are locked in cages. Normally you rescue them and add one to your army. However if you are in a raid group you can rescue them, and since you don't get any progress in a raid group, the cage will instantly reset. Allowing you to keep releasing an army of Sylvar.
The sylvar can only be taken around the quest area, unless you were to finish the bonus objective. Then you are able to bring them outside for the few remaining minutes of their duration (they seem to not be bound to the location anymore after completion).
Their total duration is 4 minutes once freed from a cage.
Keep in mind this is fixed by the time the video has been uploaded, it would be a bit too devastating on the servers if everyone were to run around with an army of 300 Sylvar...