My experience has been that in order to throttle outbound bandwidth, they just start dropping outbound packets on the floor. Unfortunately they don't seem to prioritize ICMP so acks die, causing inbound data to be retransmitted. If I exceed outbound limits, my inbound speed goes to hell.

What I'm seeing is actually a sudden, complete and persistent (lasts until power cycle) death of all traffic in both directions when the modem decides to go titsup. I can't even get to the modem's built-in web server when it's in this state. I hope that isn't by design.