FAST Search for SharePoint uses search scopes in a somewhat different way than SharePoint Server search. The FAST scopes are query-time only, and you can also specify rather complex search scopes using the FAST Query Language (FQL).
You can use search scopes to impact ranking in queries. Let's say you want to boost certain items based on their metadata, and you only want to do that for certain queries. It is possible to create a search scope for this purpose.

In the following example you will create a search scope that gives a boost of 5000 rank points for items that contains 'technology' in a managed propety 'articlegroups'.
The FQL expression you use in the scope definition is:

The aim of the first argument, "meta.collection:sp", is just to have a property expression that is always true. In this case, it will match all items that resides in the content collection named "sp", which happens to be the default content collection in the system and in most cases the only content collection). The XRANK operator requires a first argument that must match in order for the XRANK to be evaluated. For details, see XRANK syntax.

When you use this search scope in a query, the resulting query will be something like this AND(string("<user query>", Mode="AND"), xrank(meta.collection:sp, articlegroups:technology , boost=5000, boostall=yes You can add the search scope using PowerShell:

More on using search scopes: