Very simplistic answer.
Every router has an input buffer where it holds data until it can be upload or download to either the internet or your PC.
You PC will upload to your router at 1Gbpx but once the buffer gets full the router will tell your PC to pause until it has uploaded enogh data to make room in the buffer to accepct more this only takes milliseconde. Meanwhile the router is reciving data at 1Gbps but because the buffer may be full it camn't pass all that data on, for a short while you may get it but then it becomes so congested you get gridlock and everything comes to halt. Connection dropped, router clears the buffer and starts all over gain.
