From Archon Arcana - The KeyForge Wiki

After the active player discards a card with a “Scrap:” ability from their hand, they resolve the card’s “Scrap:” ability. “Scrap:” abilities can resolve in the discard pile.

If multiple cards with “Scrap:” abilities are discarded simultaneously, the "Scrap:" abilities are resolved one at a time after each card enters the discard pile, in the order of the active player's choosing.

If an ability discards a card from a zone other than the active player’s hand, any “Scrap:” abilities on the card do not resolve.

KeyForge Master Rulebook v17.3 March 2024