Bukkit

Use as slow-down:
It break sharply, whatever distance is written on the launcher sign.

Use as acceleration:
It does not accelerate at all or very sharply.

Expected results:
Target velocity should be reached in a progressive rampèup-down during the distance travel.

[?train]
launch <distrance>
<targetvelocity>

boost/break should be computed to achieve a linear or bezier like acceleration