The xiaomi mi mix 3 uses a unique slider mechanism that relies on two pairs of strong neodymium magnets to keep the phone in open and closed positions and to create a "resilient" feel between them.
If you're curious about how it works, you can watch Zack on JerryRigEverything that opens the phone. You can see two magnets at 2:35, but this is only half that.
The other half is behind the screen. When xiaomi mi mix 3 is closed, they are close together, and when they open, they will be diverted. There is a tape cable that connects to the two halves through a hole in the motherboard.
Compare the mechanism with Honor Magic 2, which uses what Huawei calls the five-track butterfly slider mechanism. You can see it in practice in this split.
Xiaomi seems to have taken special care to protect cable strips and other internal components that may be affected by slider performance. Formally, it claims that xiaomi mi mix 3 can withstand 300,000 slider movements, but an independent test showed that the handset can easily handle 600,000 slider movements.