When applying power for example forward, there is always a backward component.
If you push a wall, you will also feel a backward movement of the body.
By using power up, there is the down component.
When pushing or pulling there will be always some resistance, this resistance creates a “isometric” concept in the body.
In the picture, pushing dowh while lifting up creates isometric opposing force pairs with the hara (tanden/koshi) as the centre.