Explanation/Reference:
Spatial partition pruning is similar to the pruning the optimizer does, but it is based on location and does not require a partitioning key to be specified on the input query. When a user does a spatial query using SDO_FILTER, SDO_RELATE, or SDO_WITHIN_DISTANCE, a query window specifies an area-of- interest. Normally, the minimum bounding rectangle around that area-of-interest is compared with the minimum bounding rectangles associated with geometries in a layer using fast searches into the spatial index. When a partitioned spatial index is used, each partition's index metadata includes a column called SDO_ROOT_MBR, which contains the minimum bounding rectangle around all of the data in that index partition. At query time, Spatial indexing examines the SDO_ROOT_MBR column of each partition, and if the MBR of the area-of-interest does not overlap the SDO_ROOT_MBR, spatial partition pruning will occur and the index associated with that partition will never be searched.
References: http://download.oracle.com/otndocs/products/spatial/pdf/spatial_wp10_bestprac.pdf, page 8