You teeth are literally being pushed to a new position. So if you dont use the braces for the "minimum" duration, you teeth may not be in a place where they need to be for the second set of braces to start working. So your 2nd braces are going to be pushing more and that might cause damage to your teeth/gums. Think of it like trying to do splits. If you have never done them ever before and you try to do them on your first try, then you gonna break whatever you have down there. But if you keep at it then day by day you will notice progress. The braces are like that. Each one eases you into the next stage if that makes sense.