In Windows 3.1, it is common practice for users to share a modem between an MS-DOS application and Windows. For example, Terminal and ProComm/Plus can use a modem simultaneously, with VCD switching control silently back and forth based on an idle timeout on the VM.
Windows 95 continues to support this capability. When SERIAL.VXD is using a port and a VM attempts to access it, VCD sends RESOURCE_STEAL_REQUEST to SERIAL.VXD, which returns TRUE. The VM will use the port and then go idle on it. After this, the next call from Windows to use the port will cause SERIAL.VXD to call STEAL_RESOURCE, which succeeds because the VM has gone idle on the port.