Buying a router with QOS

I am thinking of buying a router such as the D-Link DRI-868L which as a QOS function.

I can understand that the router could quite easily prioritise the traffic that it sends onto the internet, but how does it do this for the traffic that it receives?

My main QOS objectives are to ensure that streamed music and video that I receive from the Internet are not disrupted.

The QoS on routers usually works by reserving capacity for the priority tasks and as soon as it detects what you have prioritised it slows down the other traffic for you.

Of course this priority really only works across the final link that the router can control. It will not ensure priority within the ISP network and beyond

Thank you. I was wondering how it slowed down traffic coming from an ISP. I was thinking that all it might be able to do was to delay the return ot any acknowledgement packets.