In my previous post I mentioned that you can use the same search object models and interfaces to query both FAST Search and SharePoint Search in SharePoint 2010. So let's look at how you can accomplish this by issuing a simple query using PowerShell.
$site = New-Object Microsoft.SharePoint.SPSite "http://localhost"
As you can see, there is no explicit reference to which search provider (SharePoint Search or FAST Search) to use in this script. This means that the default provider on the Search Service Application (see architecture overview) will be used. So the same script can be run, regardless of search provider, to get search results..
If you want, you can explicitly set the provider. This is done by setting the ResultsProvider property on the KeywordQuery object before executing the query.
Test different query features. Create e.g. a script that queries with and witouht stemming, and compare results.