Congestioned WAN

I read about the queues, priorities and something else…

I understood that… the priority queues START working when the bandwidth is insufficient. when your request are bigger than the bandwidth available..

Then.. i have 2 questions.

1.- how the routerOs does now when the bandwidth is insufficient?
2.- For example, I have 20 Mbps download speed, and my rules are for 20 Mpbs, with CIR, queues and etc etc etc. But, in the Afternoon, peak time (for Satellite connections), I am in the HEAVY USERS groups, and they applied me the FAP (Fair access policy) and I get few bandwidth. Then… The priorities, queue types and more start working… but, how does it know about it?

Thank you