The Expansion Pack for Fzzzt adds material for a 5th and a 6th player and 8 cards for the conveyor belt deck.
So the exansions consists of:
4 starting cards for the 5th player
4 starting cards for the 6th player
8 conveyor belt deck cards: 2 production units, 1 Fzzzt, 1 robot upgrade, 4 robot cards
In a 5-6 player game there are only 4 auction rounds instead of 5
(12 cards in the conveyor belt rather than 8 in the standard game)
In a 2-4 player game one can choose a longer game (6 auction rounds instead of 5) by adding the 8 additional cards to the conveyor belt deck.
There are no important changes in the rules.