If it's a boolean you just put the VI inside a case and wire the boolean to the case selector...
If it's another type of control e.g. a numeric you can either use an event structure and add a value change event case to it where you put the sub-VI or you can store the previous value in a shift register and check whether the current value differs from the previous one, if it does send the true to a case selector and have the sub-VI in the true case...
