How to download/extract half a file

You have to download whole file to extract part of its contents. It's same with split archive (.rar, .r00, .r01 ...) If you download 99% of file, stop download, and try to extract it, only thing you can get is corrupted file error.

It's just like tear some pages from really large book. If you want to get some pages from volume 3 of it, you have to get whole book first.

But, if they provide part of file - like vol1.zip, vol2.zip. In this case, it's like tear pages from set of books. If you want to get some page from volume 3, all you need is just take one book from set.