now look at the human body, if you contract your entire backside, wouldn't that create a similar push to the front? and vice versa? so, the question is how do you train your body to be a balloon? this is one variable of the equation.
This "push " has a proper name -- and calling things by their right name is always helpful.
Start at page 20
Note the relative contraction of the rear face of the spine and pay particular attention to the difference it makes in the direction of the red arrows on p. 22.
A great site
to explore BTW -- for excellent illustrations.