Let's look more in detail at a typical FAST Search Server for SharePoint 2010 deployment, and in particular at the Search Service Applications (SSAs) in such a deployment. SharePoint 2010 with FAST Search as the search provider will typically have two SSAs; the FAST Query SSA and the FAST Content SSA. Note that these two SSAs are differerent types of SSAs, i.e. of type Regular and ExtendedConnector respectively (useful to know if you want to script and automate setup). 

The main reason for having two types of SSAs is the separation of content; People content is indexed and served by the SharePoint Server, while all other content (documents, sites, pages, etc) is indexed and served by FAST Search. Here's what their respective responsibilities are;

• The FAST Query SSA has two main responsibilities: 1) serve all querying (against both FAST Search and SharePoint Search), and 2) crawling people content. This SSA will route people queries to SharePoint Search, and all other queries to the FAST Search query service. The queries are routed based on default search provider, a property on the SSA, or by explicitly setting a provider on each query request. Note that the FAST Search query service location is one of several FAST specific properties on the SSA.
  
• The FAST Content SSA (aka Connector SSA) is responsible for crawling and feeding content to the FAST backend. It will be configured (in Central Admin) with all content sources, except the people content source which must be setup on the FAST Query SSA. The content SSA will know where to feed crawled documents through the extended connector properties (like where the FAST content distributor is).

The figure below gives an overview of where the SSAs fit in the search architecture.

You can of course use PowerShell to list and update settings on the SSAs, e.g.:

• List all FAST properties on the FAST Query SSA:
• > Get-SPEnterpriseSearchExtendedQueryProperty -SearchApplication "Search Service Application"  
• Set the default provider on the FAST Query SSA: 
• > Set-SPEnterpriseSearchServiceApplication -Identity "Search Service Application" -DefaultSearchProvider "FASTSearch"

You can find more details on the SSAs in the Deployment and configuration of FAST Search Server 2010 for SharePoint (Beta) documentation. More information on the Service Application Framework (SAF) can be found on MSDN, and in several blog posts here.