This trick is useful when you're sharing your Internet connection among your friends or family. With this trick, you can perform such apt operation (install or upgrade) without interrupting the other members of your network.
In case, you want to limit your apt-get download speed at 20kb/s, so you should run the following command to perform apt-get operation (install or upgrade):
sudo apt-get -o Acquire::http::Dl-Limit=20K install package_name
Please take a look at statement "Acquire::http::Dl-Limit=20K", replace "20K" with your desired download speed. This trick was tested and worked well on Ubuntu 11.10 Oneiric Ocelot Beta 2, but it should works on other versions of Ubuntu too.