Q:

crouches and bends down

crouches and bends down
I see in dictionary that crouches means "To stoop, especially with the knees bent", and stoop means "bend forward". Now my question is crouches already means bend down with knees bent, so why there the sentence also has "and bends downs" following crouches? Isn't it an unnecessary duplicate?
I think crouches means changing from standing position to the below image. Correct me if I'm wrong.

Is there any different word to mean same what crouches means but it also mean how fast or how aggressively one crouches. I looking for a word which mean "crouches down very fast" 

A:

Crouching is different from "bending down." The picture that you have provided is of someone crouching, with his center of gravity in the same vertical plane as it would be were he standing. A person who is bending down would bend be leaning forward with his center of gravity ahead of where it would be were he standing.
To "duck" might imprecisely mean to crouch very quickly, but it could also mean to bend over very quickly or to fall to the ground quickly.

