When a monster your opponent controls activates its effect: You can send 1 face-up "Blackwing" monster you control to the GY; negate the activation, and if you do, destroy it, then, you can Special Summon 1 "Black-Winged Dragon" from your Extra Deck. If you control a "Blackwing" Synchro Monster or "Black-Winged Dragon", you can activate this card from your hand.
Usually, this monster is not considered good except when facing burn decks, but because you get to summon him with Blackbird Close you essentially get a powerful beater for free.
Blackwing Synchro monsters are very easy to summon with their archetype so you will often be able to activate Blackbird Close from your hand. If you have Assault Blackwing - Sohaya the Rain Storm, you can use him as tribute and then later revive him with his own effect.