I'm not familiar with VFIO and Looking glass and last time I used a VM was years ago. Could you explain how exactly did the 'flip between high performance Linux

I'm not familiar with VFIO and Looking glass and last time I used a VM was years ago. Could you explain how exactly did the 'flip between high performance Linux and high performance Windows instantly with the flick of a hotkey' actually work? What GPU setup did you have? Was the 'high performance Linux' in your case the host OS, or was it rather just another VM?