The EnableHMD node calls the function bool UHeadMountedDisplayFunctionLibrary::EnableHMD(bool bEnable) which does the same “IsValid” check you have in your if statement. If this succeeds it then does two things; first it calls GEngine->HMDDevice->EnableHMD(bEnable); to enable the headset. The second task makes a call to either GEngine->HMDDevice->EnableStereo(true); / GEngine->HMDDevice->EnableStereo(false); based on the checkbox of the node which enables / disables stereo rendering respectively. Adding only the line GEngine->HMDDevice->EnableHMD(true); to your code does in fact re-enable the headset without re-enabling stereo rendering. Stereo Rendering is enabled when your HmdWorn function calls the EnableHMD blueprint node, which allows for seamless switching of visual input to/from the HMD device when it is worn/removed.