I think the key may be in "moving together 上下相随" and "moving continuously 绵绵不断".
To keep moving, and keep moving as a whole. With the power from the legs, transmitted through the torso, manifesting in the hands, I think it means that power generated in all parts of the body, moving towards the same direction, eventually reaches the contact point (usually the hand, but not always).
So if every part of the body from the leg upwards is generating force in the same direction, that entire sum total of the force can be made to act on the contact point.
If any part of the body is stiff (not moving), it becomes deadweight. Any force generated before that will need to first be used to move that portion of the body, before any left over force can reach the contact point. I guess that is why taiji tells us not to be stiff, because any part of the body that is stiff will mean force is wasted in moving it, reducing the total force that can be brought to bear.
And the key to learning how to keep moving, and move as whole, is to practise slowly. Because it is not human nature to keep moving as a whole. Only by practising slowly can we force ourselves into the habit of moving together as a whole. But once we form that habit, we become able to move together as a whole, fast or slow. And that is when you can push effortlessly.
Or so I think...
Back to practice, and more practice.