They move everything that isn't foreground to an efficiency core, which is awful for compiling or video processing.
There's apparently a BIOS option that will use ScrollLock for disabling the efficiency cores entirely.
This is front of mind for me since reading a Cloudflare blog regarding AVX-512 instructions invoking dynamic frequency scaling to manage power/thermal capacity on chip. (https://blog.cloudflare.com/on-the-dangers-of-intels-frequen...)
If this is happening on Xeons, it's probably happening on consumer dies as well, in addition to other non-obvious power/performance optimizations. Perhaps this is why Alder Lake is pumping up the TDP[1]?
Windows has had that (foreground boost) for a long time, Intel probably piggybacks on it. It'll be interesting to see how it will behave on Linux, which AFAIK never had that mechanism (except perhaps on Android).