How to understand the "Burst Threshold" :(

I read a few lines about the Burst Threshold and rates.. but actually I can’t understand how to do it…
speed.png
I want the user to surf the internet with 1mb speed, and if he still use the full speed for 20seconds the speed reduce to 512k.
Thats it>>> (or some thing close!)

Rate limit: 512k
Burst rate :1M
burst thresold: = 384k
burst time = 20s
min rate: 64k (minimum guaranted to client)
priority: 5

watch this:

https://www.youtube.com/watch?v=pQVs9qs3Qjk

and this:

http://www.ip-pro.eu/articles.html


M.