Royce de 5’9’’ and Fredo Starr of Onyx both describe Jay-Z’s emphasis on flow in the book How to Rap – Fredo says that Jay-Z is “a master of the flow—he can flow fast, he can flow slow”. The book describes how Jay-Z uses ‘rests’ to provide structure to a verse and how he uses ‘partial linking’ to add more rhymes to a verse.
So, what is “flow”?
“The rhythms and rhymes” of a hip-hop song's lyrics and how they interact. 'Flow' is also sometimes used to refer to elements of the delivery (pitch, timbre, volume) as well, though often a distinction is made between the flow and the delivery.
Staying on the beat is central to rap's flow – many MCs note the importance of staying on-beat. MCs stay on-beat by stressing syllables in time to the four beats of the musical backdrop.
Poetry scholar Derek Attridge describes how this works in his book Poetic Rhythm – “rap lyrics are written to be performed to an accompaniment that emphasizes the metrical structure of the verse”. He says rap lyrics are made up of, “lines with four stressed beats, separated by other syllables that may vary in number and may include other stressed syllables. The strong beat of the accompaniment coincides with the stressed beats of the verse, and the rapper organizes the rhythms of the intervening syllables to provide variety and surprise”.