Does diving after a take-off help gaining altitude? I have often read fictions about aircraft, where a pilot does an action that I cannot explain:

During a takeoff with an heavy aircraft (four motors for example), the pilot rolls to the end of the airstrip, and starts to climb.
Then he says: I am too heavy to climb enough in order to avoid the mountain/trees/whatever obstacle in front of us.
So the pilot starts to dive to the ground in order to gain more speed, and then he climbs again and is able to have enough altitude.

How do you explain this action?

Considering the drag, why does the dive gain enough energy to go to an altitude above the one reach by the take off?

I might not be clear, please don't hesitate to ask details.
Here is a schema:

    Not enough         /Climb enough
        /  \          /
strip  /    \        / 
              -------
When an aircraft can barely climb after take-off, it may be flying on what is called 'the backside' of the power curve. 

Flying at very low speed:


costs a lot of power because of high induced drag
means that the engines are inefficient because they are optimised for higher speeds.


As a result, the excess power available, that is the difference between the required power and the maximum available power, is very low when the aircraft flies close to the minimum flight speed. And the excess power is what is needed to climb the aircraft. 



When the aircraft is flying below the speed at which the best angle of climb is achieved (Vx), it will be able to improve its climb angle by accelerating. To do so, the aircraft needs to use available power to accelerate, instead of using the power to climb. 

Stopping the climb or even descending a bit will convert the available excess power and optionally height (potential energy) into speed. While this initially does not give extra clearance above the obstacle, the available excess power increases drastically. This in turn gives the ability to climb at a much steeper angle, allowing the aircraft to clear the obstacle.