Flying 60s and 90s top end speed development drills focus on developing top-end speed. This is maximized by starting from a jog and ending with a jog. By the time you get to the 20-30 yard mark, you should be sprinting as hard as you can and sustain that sprint for the next 20-30 yards. As you cross the second 20-30 yard mark, the deceleration phase is activated by elongating your strides as you slow down. Do not come to a sudden stop.
To review, the first 20-30 yards are used to build up to full speed, the next 20-30 yards are used to sprint at full speed, and the final 20-30 yards are for decelerating using long strides.