What is the power of our will?

The will power of the spirit is the motive power with them. 

It is not mechanical at all, so that if a spirit were in the same house as his earthly relations, and he found it inconvenient to be constantly in their way, he would naturally wish that the room were removed—or the inconvenience remedied in some other way, and in proportion as he exercised his will power, and to the strength of will he brought to bear on it would accomplish this object and effect the alteration required in a more or less intelligent manner. 

In short, will power is to the spirit what mechanical force is with us and is the force by which spiritual matter is displaced.