Speed negotiation?

Please help me understand how a device (PC or router, in this example) knows to slow down broadcast of traffic.

Say my cable modem from my ISP is coded to only allow 50mbps. My internal network supports the normal 100mbps.

How does my router know to not send my cable modem more than 50mbps of data (because anything above, I assume after the cable modems RAM fills, will be dropped).

I know TCP/IP has "Autonegotiation" which determines link type, but it's only based around if t's 1000/100/10 ethernet. No custom speed variable, such as 50mbps in this case.

Say I put my network uploading data maximum stress to my internet connection, at 100mbps. Will my cable modem ever send some special packet telling me router that's too much or something?


edit:

I suppose if I knew this, I could also understand how when I upload a file to another computer on my network, it does it at the full 100mbps, but over the internet, limited to my upload speed; but how does my computer or router know to slow down because my ISP doesn't allow 100mbps up? It has to be able to detect link speed somehow, otherwise just constantly sending 100mbps up will result in tons of lost data at my router/cable modem, and give the computer a frequently false sense of upload speed.