You are legally required to be in the country at the point that the Home Office receive your application. To ensure this happens the NCS service normally say you shouldn't travel for 5 days after submitting your papers just to be sure that if there is a delay in the post etc that you are still here when it hits the HO.
The important point here is, that it's the date the Home Office's receive the application – not the date that you submit it via the NCS or send it thru the post.
