When he got locked up, all he had left was his thoughts and his rhymes. This might be a subtle reference to the saying “When God closes one door, he opens another”; the first door being the cell door, and the second being his mind.
Also, the part of his brain that was “frozen” was the rhyming/writing side. It was frozen because he was concentrated on the drug game. Freeze is slang for cocaine.

Also, note the alliteration of the letter f with “forward”, “four”, “forced”, and “form”.