The Xiaomi Mi Mix 3 uses a unique sliding mechanism that relies on two pairs of strong neodymium magnets to hold the phone in the up and down positions and also provide the springy feeling in between.
If youre curious about how that works, you can watch Zack from JerryRigEverything take the phone apart. You can see two magnets at 2:35, but thats only half of them.
The other half are behind the screen they line up perfectly when the Mi Mix 3 is closed and push each other away when it is open. Theres a ribbon cable that hooks through a hole in the motherboard to connect the two halves.
Compare that to the Honor Magic 2, which uses what Huawei is calling a butterfly five-track sliding mechanism. You can see it in action in this teardown.
It looks like Xiaomi took special care to protect the ribbon cable and other internal components that may be affected by the sliding. Officially, it claims the Mi Mix 3 can withstand 300,000 slides, but independent testing showed it can easily do 600,000 slides.