Value search queries could potentially contain many results. You can use RestrictToProperties attribute to limit the number of returned results to a list of one or more specified attributes.
If a managed attribute is searched, using RestrictToProperties you can search within a whole managed attribute and its entire hierarchy of values, but you cannot restrict value search to a subtree within a particular root value in the hierarchy.
<ns:ContentElementConfig ... <ns:RestrictToProperties> <ns:Property>ProductCategory</ns:Property> <ns:Property>Bike Racks</ns:Property> </ns:RestrictToProperties> </ns:ContentElementConfig>