Complex sentence: Specifically, he is known for being able to rap "off the top of the dome" anywhere at anytime, as evidenced when he did a freestyle from his hospital bed after a car accident.
Split into simpler sentences: Specifically, he is known for freestyling, or rapping "off the top of the dome" on a whim, that is he is able to do it anywhere at anytime. This is evident when he did a freestyle from his hospital bed after his car accident.