Backlinks in Hypermedia

Backlinks is the mechanism documents can identify where are being linked from. Think of it as the other direction from where a link is pointing to. Backlinks is what makes Bidirectional link possible

How to create Backlinks in Hypermedia?

There is no way to explicitly create backlinks. backlinks are computed from the links you create (or are created) inside each Hypermedia node.
When you create a reference inside "Document B" to "Document A" (a reference can be a link or an embed), Your local now add this link to a local index of all the cross-references between all the documents you have in your node.
This makes it possible then to ask for all the "Citations" for a particular document (Citations is the name we use to describe the documents that include a reference to another document).

Backlinks or Citations?

We tend to refer to both as the same thing but I believe there's a subtle difference: Backlink is the direction in which the reference is pointing to and Citation is the "actual" thing that makes backlinks possible.