When a router wants to signal congestion to the sender, it adds a bit in the header of packets sent.
When a packet arrives at the router, the router calculates the average queue length for the last (busy + idle) period plus the current busy period.
When the destination replies, the corresponding ACK includes a set congestion bit.
Note that this technique does not allow for effective use of the line, because it fails to take advantage of the available bandwidth.
Besides, the fact that the tail has increased in size from one cycle to another does not always mean there is congestion.