Check for folder date while another app is running?

Question

So, I have a program that keeps backups from another app and I want to create a backup whenever the files on a specific folder are modified by the other app. This must be done while the program waits the other to close.

I have the following questions:

1. Is it possible without wasting the processor's cycles too much?

2. The code for launching the app goes like this:

Hide();
                //Any arguments for the executable (set in this executable's code) are loaded.
                //So, for example you pass the arguments --LoadFile <file> via code, this will happen:
                // Run "<Program Folder>\Program.exe" --LoadFile <file>
                // Set these with _softProcess.Arguments = "argument1 argument2";
                _softProcess.StartInfo = _softInfo; //Get the arguments from the variable _softInfo
                //Start the program.
                _softProcess.Start();
                //Wait for the game to close before closing Northbridge.
                _softProcess.WaitForExit();
                //Calls the Auto-Backup code.
                if (Settings.Default.AutoBackupEnabled) AutoBackupCode();
                Close();

How do I set it up?

EDIT: I forgot to mention, the app can access and modify multiple files.