Entity falling distance is the distance the entity have fallen without reaching a block.
It can be used to calculate different damage from falling, or activating an effect after a big fall.
Retrieving the falling distance
float distanceFell = entity.getFallingDistance();
Setting the falling distance
This can be used to simulate a different falling distance than the real one. Bukkit will calculate the damage using the new falling distance.
entity.setFallingDistance(float distance);