ESA & savings

So, I'm curious about how this works with ESA and savings. So once you go over the £6K you notify them as to the amount -say £7K and thus you get £4 deducted weekly? But what happens if you're wanting to regularly save but at the same time you're withdrawing money for maybe large purchases so the amount fluctuates - do you literally have to contact ESA every time the amount then changes or how does it work exactly?