When jump on heap of sand.
If a man falls on a soft landing site like heap of sand, the man takes a longer time to come to stop. The rate of change of momentum is less due to which a smaller stopping force acts on the man and he does not get hurt. Thus, Sand, being soft reduces, the man's momentum more gently.
When jump on heap of sand.
If the man lands on hard ground like cement floor, then his momentum will be reduced to zero in, very short time. The rate of change of momentum will be large due to, which a large opposing force,will act on the man. This force, can lead to serious injuries to the man.