Hi,
The fullback is through the line.
The fullback is through. (This also has another, informal meaning, ie The fullback is finished, fired, has lost his job) but I don't think that is what you are asking about.)
This just seems like ellipsis to me. 'Through' is still a preposition. It's like
1. He walked across the road.
2. He came to the road. He walked across.
It's OK as long as I understand what is omitted.
Best wishes, Clive