This first explains the power of Firedog and also Blackwing Shura. Firedog with 1900 attack is big enough to attack over most monsters while Shura may need help from Kalut:
http://go-ygo.com/yugioh-card-design-plight-small-monster/
This next earlier posted on the powerful first turn play made with Gravekeeper's Spy:
http://go-ygo.com/yugioh-quick-hit-turn/
Ryko is also another not bad first turn play but it pales in comparison to Spy into Descendant, labelled as "probably the strongest opening turn play". Interestingly, Spies are also a very good answer to Firedogs. (Shura can get help from Kalut)
Now, put the 2 blog posts together and they explain the strength of Flamvell decks!