The long version is: Whenever one or more creatures you control deal combat damage to a player, you may sacrifice one of them
Unfortunately, in reality, despite its pretensions of being an Energy deck, it is much more an artifact beatdown deck with some Energy support
Along these same lines, the thickness of a double-sleeved deck means that it won't fit in your average deck box
Computer Search, discard 2 energy, grab whatever, psychic energy on Mewtwo, Absorption, you just threw down 3 energy in one turn and were ready to blast for 40 next turn to anything that wasn't Wigglytuff and 80 to Hitmonchans that threatened your Wigglies
Double-faced cards were hence an infrequent thing