When your opponent draws the face-up card, immediately send it to the Graveyard
Whenever a vigilance creature attacks, it doesnt become tapped
This makes him incredibly easy to build around, given the sheer number of +1/+1 counter support cards available in the Commander format
Both Burning-Tree Emissary and Llanowar Tribe have some additional upsides as well, with Burning-Tree Emissary adding two mana when it comes into play, which means we can cast any number of Burning-Tree Emissaries to up our devotion to green and still have enough mana left over to activate Nykthos, Shrine to Nyx at the end of the chain
Also, it can trigger the effects of Armageddon Knight and Star Seraph Scepter during your opponents turn
It turns out that being able to reuse instants and sorceries from your graveyard is just really good, especially when you have the option of doing it with a flash creature