system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If my ISP has a cable that can provide upto 50mb speeds, but i purchase the cheapest package (only upto 10mb) how do they ensure i don't get the fastest speed available?

This thought occured to me recently since my dad has been comparing ISPs since we are looking for a new provider. A quick check on BT told us we can get speeds upto 16mb since a new higher speed cable has been installed in the area. However if i were to buy the lowest tier package (speeds only upto 10mb or something) whats stopping me from actually getting the full speed. 

Is it as simple as some software they have that automatically caps my speed somehow? How does one do that if so?
gpt
When someone sends some data to you, they send it in little chunks called packets.  Usually they start sending packets fairly slowly, and increase the speed until your computer is like "wooooah there hold up buddy" and then they back down a bit and keep sending at that slightly reduced speed.  Your ISP limits your speed by having your modem do the "wooooah there" message at 10 mbit/s or whatever your limit is.  It's possible to hack a modem to get around that though, but the technique varies a lot and may be detected by your ISP.