I quickly hit ctrl and shift like everyone else but she always stops halfway or gets blocked by a cube

So I tried double-clicking shift and found it easier to succeed, but I don't know why

The first two times are only one shift and the last one is double-click shift

(Google translate)

There needs to be a bit of a delay between the two inputs. If they are input too fast, you don't go very far. If you give a slight delay though, you allow six to get far enough through the grab that the second input gives her a huge slide. When you do the second input too early, the part of the grab animation being canceled is different and the result is the shorter slide like what is seen in your video. I recommend watching the world record by Sai. He uses input capture so you can see the exact timing between the two inputs. If you need more help there is a great community on the discord who can help even more than I. Best of luck practicing the zip : )