How does a content locker postback work?

I never understood this part of the content locker. I read about the 'postback' and I think I can understand the concept of it, but I have never seen on a sponsor where it lets you enter a postback URL or anything of the sort. I assume it's in the way you script it.

Not looking for a code here just an explanation or tip to get me going if that's possible, thanks

So it seems the postback is setup by the sponsor. The reason I haven never seen it is because I was looking on non-incentive networks.

If anyone has anymore input it would be appreciated, but I think I just answered my own question.