In the push mode, the LCM expects all DSC resources to be present on the target node. Since the LCM doesn't have a way to know where to get the DSC resources used by the MOF file, it can't get them for you. Before running any push deployment on a target node, you must put all DSC resources needed there first. If they are not present, then the execution will fail.

Using the Start-DscConfiguration cmdlet, the MOF files are executed immediately. This kind of execution happens only when the user initiates it. The user can opt for the execution caused by the Start-DscConfiguration cmdlet to happen interactively and see the output as it happens or have it happen in the background and complete without any user interaction.

The execution can happen again if the LCM ConfigurationMode mode is set to the ApplyAndMonitor or ApplyAndAutoCorrect mode, but it will be applied only once if ConfigurationMode is set to ApplyOnly.