It doesn't do that. Whatever seems dependant gets lumped into one thread and limited to a single core. Fortunately Ableton cannot limit what plugins do in their own runtime so as you say U-HE multicore functionality works just fine and jBridge can run plugins off of separate processes. Not sure why people are being defensive about it or attack for pointing this flaw out. It's great that you think Ableton works as intended for you, but once you go beyond Live devices and use more professional tools, you'll hit the wall. People who I know hit this as well just moved to Reaper, but I cannot get on with its workflow.