Cause that ain't good enough, you expect me to fold myself in half
'til I snap,

from Eminem – 25 To Life Lyrics on Rap Genius

Meaning

He is using another way to describe the terminology of “bending over backwards”.

He is expected to “bend over backwards” for you, to the point where he snaps; it doesn’t matter how much he does because it’s not good enough and more is always expected from him.

Snap serves as a double entendre

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