The whole idea is to have a longer, more gradual acceleration during the leadin than you might otherwise, and reach a higher Max Velocity.
You don’t accelerate as hard as during a race, but you want to reach maximum velocity at the start of the 20 meter section and maintain that velocity through the 20 meter section.
The length of the leadin depends on fitness level, with faster people needing longer leadins to reach maximum velocity, and slower people not being able to handle longer leadins without slowing down (already past MaxV).