I have a doubt, I remember Google treated subdomains as new domains but I have seen many websites that share the Pagerank between the main page and a subdomain for example

whos.amung.us PR7
amung.us PR7


and the owner doesn't use the main page, so how is he doing this? what type of redirection is he using?

If I want my subdomains to grab a big amount of pagerank from my main page what should I do? link to the subdomain from my homepage and link back to the hompeage from the subdomain?