Question

why a person flexes one’s knees while jumping from a height.

Answer 1

absorb the impact...

Answer 2

@riyadh firstly it is a physics question secondly he flexes his knees to increase the time contact so that the impulse will be small otherwise his bones will break:)

Answer 3

while you might want to post it in science...
by flexing his knees he get to reduce the impulse force on his legs and create an arc so that the force is dissipated over the thicker bones in the thigh.