Advanced format ruling
Boss Rush
Card effect (current official text)You cannot Normal Summon or Set. During the End Phase of the turn that a "B.E.S." monster or a "Big Core" is destroyed and sent to the Graveyard, you can Special Summon 1 "B.E.S." monster or 1 "Big Core" from your Deck.
Current rulings for this card are maintained by Konami.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Advanced forum — sourced change requests get reviewed.