So looking at this picture- the 'up' stride is pictures 6 to 1 (mainly focusing on picture 1). Here the back is angled upward, and theoretically the horse is exhibiting freeness through the shoulders and weight is carried on the hind.
Sometimes, people will drive their seat down during this part of the stride, and then backwards during the following strides 2,3 and 4. If your pelvis is there locking down the horses shoulders, they cannot exhibit that upward, freeing that is seen in their natural gait. By remembering to scoop the lower abdomen forward and up with Photo 1, you can allow the horse to do what it needs to do.
If I can describe it differently: the correct way would be to scoop the pelvis forward, up, back, and then down consecutively with pictures 5 (forward), 6-1 (up), 2-3 (back) and down (4). I visualize the pelvis making a counterclockwise oval if viewed from the right side. If you imagine doing this on the horse in the picture, it actually moves you just as the horse is.
The pit that some people fall into is moving the pelvis down, back, up, and forward consecutively with pictures 1-6 (down), 2-3 (back), 4 (up) and 5 (forward). Essentially creating the backwards oval from the above explanation. I suspect the error stems from the number 5 stride, where the rider could easily slide down and forward in the saddle as the back is angled in this way.
Anyhow, it is honestly such a hard thing to put into words, so I apologize if I have just confused you further. It's really just a feel thing. Ask your instructor if you can focus on it in your next lesson, then they can be right there in the moment to tell you when to do what.
Hope this helps, and apologies, Faye, didn't mean to hijack your thread.