It doesn't matter if Green Goblin was on the battlefield at the time the cards were discarded
Its as if Yawgmoth, Thran Physician showed up to let black decks do everything at once
That's a very good advantage, and the fact that it's so cheap to play means you can do a massive amount of setup early in the game, or just a play a big monster and have it get over its summoning sickness before the opponent can do anything whatsoever
They're double sided cards that can transform
Heres a simple example, where I put a couple of play tables just beside the windows: Mod #4 Fast Workbench As youve probably already guessed, this mod lets you configure the speed in which you pack cards at the workbench
You draw a card and discard a card every time shes tapped, and with enough lesson cards in your graveyard, all noncreature spells cost one mana less