Well it’s basically you general “limp-mode” where the ECU basically bypasses the turbo in an effort to prevent engine damage, there’s more to it, but that’s the basic version. Your problem is that there any number of reasons that the car could be doing this, and there is really no way anybody can help you without more information. You need to get the codes read. That will help to point to what’s causing the issue. You can go to AutoZone to get them read, but they cannot read all of them. As an example, when I was having an issue before I bought a scan tool, I took my R60 to AutoZone to have the codes read while the car was actively in limp-mode. Their scanner showed no codes at all. But when I took it to a shop, it showed 5 codes that AutoZone couldn’t read. There are some codes that need specific BMW software to read. A mechanic that works on Mini and BMW should be able to read them, or scan tools are available to buy, or get them read somewhere.
After you get the codes, post them up here, and folks will be able to help from there.
Good luck