For many years SEO experts were happy with two way linking or reciprocal linking but in due time researchers anticipated a change in Search Engine's Algo.

Not before too late it happened and the value of reciprocal linking started decreasing, it can never die but was almost like a dead. Now only escape was one way link, and getting one way link was close to impossible, possible ways were

Directories.

Paid links.

Provide info capable of fetching some links.
Nothing was as easy as reciprocal links, so all the community who were used to reciprocal links thought of an alternative. It is often termed as three way links.

My Site B (secondary site) links to your site X and let X link to my main site A.

B --> X
X --> A

Now if I take this as two, first
B --> X , mysite B is linking is your site X , it should be consider as one way link , so cashing the one way link points.

Again if I consider
X--> A , where your site X is linking to my main site A , also considered one way link in sight of google.

QUOTE

Always Google and other SEs have adjusted their algo according to SEO tricks and SEO tricks according to SEs algos.

Google's probability of catching you is very less but not 0, google can catch you by pattern recognizing methods , almost all incoming links to A is given a link from site B, that's the catching pattern. Its almost impossible to scan million of sites to check for such pattern , but then how Google reaches to site B, HOW?

How Google reaches to site B through site A

Through whois

If both sites have similar whois info, then B is reachable through site A reference.

Same C Class IP

If both the site has same C class IP (111.111.111.abc and 111.111.111.xyz is same C class IP) , Same C class IP often means same server, and many amateur SEO experts do this mistake.

Related Site Syndrome

If Page A is linking to Site X and to Site Y , similarly Page B, Page C , Page D … (some good number of pages) links to both the sites , then you do a G or Y search with related:http://Y.com/index.html, you will get Site X as related (even though X and Y may be extreme apart with theme) and similarly if you do G or Y search for related:http://X.com/index.html you will get related Site Y, thus Search engine can understand that Y and X are related.

This is not a proven theory , but a happening thing as manual checking is gearing up, once big mamma said precaution is always better than cure, be careful to enjoy the investment on your site.

Jimmy Spencer