Summoning Clock is a Trap Card used by Atem during his Ceremonial Battle with Yugi Mutou.
For it to be activated, the opponent must use a Spell or Trap Card during the player's turn. When activated, the player can cancel the Battle Phase (meaning that any battle will be nullified) and, by tributing a monster, summon as many monsters as the turns the card was face down on the field for. Not only that, but if the player performed no Normal Summons in these turns, the Summons made through the Trap Card will count as Normal Summons; and, if the player pays 1000 Life Points, he has as many Tributes as the turns mentioned.
Atem used this card to summon all three God Cards on the field against Yugi. Having summoned Obelisk in the previous turn, Atem activated the Trap when Yugi used Time Jump to raise Silent Swordsman's ATK. Since three turns had passed, Atem could summon three monsters by tributing his Tricky Token. The first two monsters to be summoned were King's Knight and Queen's Knight, which allowed Jack's Knight to be Special Summoned from Atem's deck. For the last summon, Atem paid 1000 Life Points so that he could use three sacrifices: thus he summoned the Sky Dragon of Osiris. Since it was Atem's turn, he could still do another Normal Summon: by tributing the three knights, he was able to call out the Winged God Dragon of Ra. By ending his turn, Atem also undid Yugi's strategy to weaken Obelisk, returning its ATK to 4000.
Thus Yugi had: Silent Swordsman (ATK 2500), Green Gadget (DEF 600), Red Gadget (DEF 1500) and Yellow Gadget (DEF 1200), with a face-down card. Atem had: the Giant God Soldier of Obelisk (ATK 4000), the Sky Dragon of Osiris (ATK 2000), the Winged God Dragon of Ra (ATK 5000), with a face-down card.