When Immortal Phoenix dies, it and Rancor trigger.
Resolve the Rancor trigger, returning it to your hand from the battlefield.
Resolve the Immortal Phoenix trigger, returning it to your hand from the battlefield.
Resolve the Composer of Spring ability from step 2, putting Immortal Phoenix onto the battlefield from your hand.
Repeat.
This clever loop relies on the seamless interplay between self-returning permanents and a reliable sacrifice outlet. By enchanting Immortal Phoenix with Rancor and sacrificing it to Phyrexian Altar, you generate a steady stream of mana while sending both the Phoenix and the Aura back to your hand as they die. Meanwhile, Composer of Spring watches your enchantments enter the battlefield to effortlessly cheat the Phoenix right back onto the board for free, ready to be sacrificed all over again.