Understanding Storm Internals by Examples - Real-time Analytics with Storm and Cassandra - page 66

Direct grouping
In this kind of grouping, the Storm framework provides the ability to the sender
component (spout or bolt) to decide which task of the consumer bolt would receive the
tuple while the sender component is emitting a tuple to the stream.
The tuple must be emitted to the stream using a special emitDirect method to the
stream, and the task of consuming a component has to be specified (note that the tasked can
be fetched using the TopologyContext method).