There's power - as in watts - and there is force - as in how hard you push.
They're not the same.
Your body weight alone will generate the most force when your center of gravity is right above the pedal.
And just like a weight lifter striving to have the bar close to his body, you'll be able to push the hardest when your body is as straight as possible between what you brace against and what you're trying to move.
Power is a different thing, basically composed of how strongly you can make a certain move AND how fast you can repeat it.
With geared bikes, the strongest push isn't always that interesting.
If you can do do the same move at half the strength but at 1/3 the time, that'll make more power than the slow-and-hard.
On a properly sized MTB you bend at the hip/waist, stretch your arms a bit and only lean your upper body forward. This keeps your center of gravity pretty much right above the pedals anyhow.
Leaning your full body forward will unweight the rear wheel and seriously hurt your ability to climb on loose surfaces.
Last edited by dabac; 03-11-17 at 04:29 AM.