The flow diagram is a system of notation to show how flow works. Every beat has 4 counts (1...2...3...4). If u want to write to a specific beat, or ride the beat in a particular way, the use of a flow diagram can help you plot this visually. Create a table with 4 columns representing the four parts of the beat. Each row will be filled by a line from your verse. Assign the words in each line to the columns to plot which part of the beat they come in at.
To give an example, here are the opening lines from the Pharcyde’s track “Drop,” from the album Labcabincalifornia, written normally:
Let me freak the funk, obsolete is the punk that talks
More junk than Sanford sells. I jet propel at a
Rate that complicate their mental state as I invade their
Masquerade. They couldn’t fade with a clipper . . .
Now here are the same lyrics, this time shown in the flow diagram: