How do Thru Lines work?

[Phoenix]

I've linked my aunt to her tree. Lo and behold, she's on Thru Lines.

It has provided a connection to an Australian whose tree is quite rudimentary, only three or so generations and no common names.

It appears to piggy-back that tree onto another to find connections.

I'm sure it's right, since shared matches all point in the same direction.

(Dodgy genealogy mean there are some incorrect potential ancestors for my aunt, btw!)