For example,
sudo ipfw pipe 1 config bw 500KByte/s
Limits all network on your box to 500kb/s
sudo ipfw add 1 pipe 1 src-port 443
To direct all traffic through port 443 through your slow pipe
sudo ipfw delete 1
When you are all done delete
EDIT: its called the Network Link Conditioner and apparently you install it from within xcode now.
EDIT2: I have just learned that ipfw is gone in yosemite? How distressing. Did they replace it with something?
I learned about it here: http://nshipster.com/network-link-conditioner/
Is a good thread to follow
Also has some handy features like AJAX debugging.