As I understand it, it is a kind of impulse or projection of will into the coming work.

I may here illustrate this with a curious fact in physics.

If the reader wished to ring a door-bell so as to produce as much sound as possible he would probably pull it as far back as he could and then let it go.

But if he would in letting it go simply give it a tap with his forefinger he would actually redouble the noise.
Or, to shoot an arrow as far as possible, it is not enough to merely draw the bow to its utmost span or tension.

If just as it goes you will give the bow a quick _push_, though the effort be trifling, the arrow will fly almost as far again as it would have done without it.
Or, if, as is well known, in wielding a very sharp saber, we make the _draw-cut_, that is if we add to the blow or chop, as with an axe, a certain slight pull and simultaneously, we can cut through a silk handkerchief or a sheep.
Forethought is the tap on the bell, the push of the bow, the draw on the saber.