Flushing dirty pages to the data pages disc

I am confused about when to flush dirty pages to the data pages disc. As I understand T-log disc and data pages disc are different from each other and when buffer pool is full and do not have enough space least recently used page is flushed to the data pages disc. And when a transaction commits it is flushed to the T-log disc but not data pages? When to flush dirty pages to the data pages disc?
Here is the link that I read from: http://ift.tt/1EfNPQa