Jeff Bass, one of Eminem's collaborators on "Lose Yourself," has said that Eminem has a unique way of writing lyrics. The words come together sporadically, and Bass notes that, if you picked up a piece of paper Eminem has been writing on, you might find a series of nonsensical words with phrases from one song followed by an idea for another. The final flow Em delivers to the mic comes together in his head and finds its sync with the music.
Eminem's own style of rap is multi-dimensional; he's a master at multisyllabic rhymes, couplets, fitting numerous rhymes in a bar, and quickly switching between varying levels of rhyming complexity along with harmonies. "Lose Yourself" features Eminem's incorporation of assonance (where two words don't have the exact same ending, but use their shared vowel sounds to make the rhyme). He uses enunciation and articulation to make the different consonant endings link together. With assonance, as long as the vowel sounds remain consistent, the consonants can be different.
In his book, How To Rap: The Art and Science of the Hip-Hop MC, Paul Edwards cites an example of assonance from Madvillain:
"The most blunted on the map / ...alley with a hood rat." Here, 'map' rhymes with 'rat' using assonance: both words have the same vowel sound in the middle, the 'a' sound. The surrounding sounds are not the same—one word ends with a p and the other ends with a t—but the vowel sounds are, and this creates assonance. (Source, 84.)
So, with "Lose Yourself," Eminem essentially "bends" his words into a fitting rhyme scheme. Here's an example in the first verse where he intercuts two sets of vowel sounds together (lyrics bolded to indicate the long "o" rhyme and italicized to indicate the short "a" rhyme):
Oh, there goes Rabbit, he choked
He's so mad, but he won't give up that easy, no
He won't have it, he knows his whole back's to these ropes
It don't matter, he's dope
He knows that, but he's broke
He's so stagnant that he knows
When he goes back to his mobile home,
That's when it's back to the lab again yo
The variation of rhyme is certainly more complex than a traditional couplet with a matching end rhyme, as is typical of old school rappers. Assonance in this case however, also contributes to the tension of the song. 8 Mile follows Eminem's character through the underground rap scene of Detroit, where performers typically have less than a minute to create original rhymes that are clear, clever, and catchy.
Eminem's ability to bend words to his will serves only to impart the mood of freestyle rapping, along with the song's larger theme of struggling through challenges.