The Advanced control is a quick and easy way for power users to write their own queries. It takes a valid KQL query and uses it to filter results from a search mask. You can even put a default value in it.
<Control Name="AdvancedSearch" DisplayName="Advanced Search" Type="AdvancedBooleanSearch" Value="mvClient:Acme" />
This simple example defaults to return items where managed property mvClient contains "Acme".
Note that the KQL entered by the user can include quotes and other characters (i.e. no need to escape them out).
Note: This control is currently only a single line so long queries expand the width of the search pane.