He uses images of spirals and falcons to reinforce the idea that anarchy has invaded the world. He does this, to show how this anarchy is repetitive but very powerful in imprisoning living beings.
We can arrive at this answer because:
Yeats portrays how a falcon got caught in a spiral that grows ever larger, to represent anarch's invasion of the world.
This spiral prevents the falcon from following the falconer, but keeps immersed in this endless cycle, attracted by its center that expands more and more.
This type of language is used to show how anarchy is a prison for people, who cannot follow their own lives, but are immersed in this environment of violence and insanity.
This question is about the poem “The Second Coming.”
