How can a website block your Referer data?

Question

I would like to understand how Yahoo Answers block the Referer on linked URLs in their responses. Ex: https://br.answers.yahoo.com/question/index?qid=20101129074952AACQRi3

Clicking on any link and checking the GET you will see that it does not send the "Referer:" property on Header. Then.. i can not monitor on my site when a click comes from Yahoo Answers. How is this possible? Is there some setting on their server which blocks the sending Referer data?

Is there any other way to identify if a link is coming to my site from Yahoo Answers?

Edit: I've found the answer..

Browsers will not send a referrer when linking from HTTPS to HTTP

When your site is on HTTPS and you are linking or redirecting to a HTTP site, the browser will not send a referrer.

Answer 1

Actually, referrer param is set by a browser, so it is client side related. There are most common situations when referrer can be dropped out such as browser plugin/tool for changing referrer or ssl security feature when browser is not to pass referrer etc.

More reliable way to track referrer is to construct target url the way it contains some kind of requestor id which will be processed on the end along with referrer parameter

Eg: example.com?ruid=1234