Intruder Alarm changes how creatures untap. They stop untapping during their controllers' untap steps, but every creature entering the battlefield untaps all creatures. With a repeatable creature-producing ability and enough mana support, that replacement rhythm can turn a small board into a powerful engine.
Kiki-Jiki, Mirror Breaker illustrates the potential. Copying a suitable nonlegendary creature creates a creature token, which triggers Intruder Alarm and untaps Kiki-Jiki. That can allow another activation and another token, repeating while the relevant pieces remain available. Mana creatures can also untap with each entry, helping pay for token-making abilities that require mana as well as tapping.
The effect is symmetrical. Opposing creatures untap too, and opponents can exploit their own tap abilities or creature entries. Without a dependable source of new creatures, your own attackers and mana creatures may stay tapped longer than expected. A deck using Intruder Alarm should be able to create entries on demand rather than simply hoping to draw another creature.
Sequencing matters around interaction. The untap trigger uses the stack, giving opponents a chance to remove a key permanent before the next activation. Keep the necessary mana and targets available, and understand which piece actually sustains the loop. Intruder Alarm is best in a list built around tap abilities and repeatable creature creation; in a generic creature deck, it can just as easily make normal untapping less reliable.
