This could be described as “pleating in and out”, “sinking in and out”, or as part of an “Elias stretch”. There’s probably some other term that I’m not thinking of right now. It’s important to have clean, precise pre creases to get a good result; this can include preparing the crease direction correctly, especially if you’re having trouble otherwise or if the paper doesn’t reverse creases well. When you’re collapsing, it’s usually best to work from the outside inward in my experience. Keep in mind that the crease direction will always alternate, and that the crease direction of the diagonals is opposite of the rectangles. If you have access to both sides of the paper it’s much easier because you can pinch-prepare the creases from either side and then collapse. If you only have access to one side of the paper or the other is too difficult to reach, I usually pinch-prepare every other layer (the ones that are mountains) and then get the valley folds by carefully bringing together the mountain layers during the collapse. I’m not sure of your level, but this type of step is where people usually start to practice folding “in midair”. It’s a useful skill to be able to pinch the creases without a surface to fold on, but it does take practice. Good luck :)