Basically on cng emulator stops your injectors from running while telling the ECU that they are working as simple as that.
Some people to save money of emulator installation just connect the cng switch with your fuel pump so no fuel for injectors to feed on when you switch to cng but this method can damage your injectors as they will be running dry. Best will be to get emulator and also connect relay with cng switch so it switches off pump when on cng.