I've generally been taught that with an opponent of equal size, you want to ground the opponent (bring them to their toes). With a taller or bigger opponent, it may be necessary to take them to their heels (Uproot literally).
Using Chi Sao as an example, bringing them to their toes would be Lop Sao'ing while uprooting would be raising their arms and advancing on the centre (cannot remember the actual term at this moment - my apologies). Of course you are not uprooting them too much that it makes the force of your punch weaker.
Someone hit the nail on the head when they pointed out that your opponent resisted the downward pull. IF there is resistence from downward pressure, the only direction to go in resistence is up.
I am a bigger guy so most people have to use this with me.
Just my opinion of course.
Kevin
Continued blessings in your life and your training.
The key to understanding is to open your mind and your heart and then the eyes will follow.