In the November 2014 CU, were included a serie of new features. One of those was to turn on/off the partial results feature on farm level.
Once this feature is enabled, in case of one of the search replicas is degraded (maintenance, goes down unexpectedly, etc.), end users will still receiving results from the avalaible search components. Also, when this feature is enabled. a new metadata property is added in ResultTableCollection.
And how to enable this feaure? Of course doing PowerShell 🙂
$ssa=Get-SPEnterpriseSearchServiceApplication $ssa.AllowPartialResults = $true $ssa.Update()
Note that this change will affect all search sites, so be aware 🙂
Hope that helps!