Managed Property and Rank

Managed Property
A managed property is content or metadata associated with an item that may be searched or used in other ways, such as being displayed in search results or used for query refinement. To make metadata searchable, first define the managed properties. Then, map the crawled properties that should be searchable to a managed property. At this point, the crawled metadata is searchable as a field. To search this metadata automatically as a general part of the item, map the content of a managed property into the default full-text index (known as content) for querying.
http://msdn.microsoft.com/en-us/subscriptions/gg982954.aspx

Ranking and Sorting (FAST Search Server 2010 for SharePoint)
Sort Search Results by Rank
If you want to use a rank profile that is different from the default rank profile, you can specify the name of the rank profile in the sort specification.
The RANK operator or XRANK operator in the FAST Query Language (FQL). These operators enable you to apply a conditional rank boosting if a specific query condition is met.

Sort Search Results by Managed Property Value
You can specify query result sorting based on the value of one or more managed properties in the query result. This means that FAST Search Server 2010 for SharePoint performs the sorting based on all results that match the query.
http://msdn.microsoft.com/en-us/library/ff394654.aspx#ranking_sorting_rank

Example:
criteria.OrderBy = new List<OrderData>()
{
new OrderData(new RankPropertyExpression("departments"),
OrderDirection.Ascending)
};
http://documentation.ektron.com/cms400/v8.60/Reference/Web/Navigating/Search/Setting_up_FAST.htm

Change Importance Level


$RankProfile = Get-FASTSearchMetadataRankProfile -Name default1
$content = $RankProfile.GetFullTextIndexRanks()|where-Object -filterscript {$_.FullTextIndexReference.Name -eq "content"}
$content.SetImportanceLevelWeight(1, 5)
$content.Update()
$content.GetImportanceLevelWeight(1)

http://fs4sp.blogspot.com/2012_01_01_archive.html