The adjustment needed is to lower your seat. Your seat is currently set for road riding where pedaling efficiency is important.
The idea when Mountain biking is to maintain the center of gravity over the Bottom bracket. As the down hill gets steeper, you need to shift weight back to achieve this, as the uphill gets steeper, move weight forward. The seat post in a normal climb position prevents the required weight shift.
The weight shift has to be done by moving the whole torso. Assume the 'Attack position' (google is your friend) - This correct riding position is arms are bent, elbows out (ideally forming a C when looking head on to the rider) and torso is relatively level, legs straight but knees not locked (i.e. very slightly bent - knees bent is common advice but gets miss interpreted).
(IMHO his knees are bent too much. This amount of bend requires a lot more effort in the quads than needed.)