can anyone explain about incremental aggregation with an example?

 When you use aggregator transformation to aggregate it creates index              and data caches to store the data 1.Of group By columns 2. Of   aggreagte columns             the incremental aggreagtion is used when we have historical data in              place which will be used in aggregation incremental aggregation uses  the cache which contains the historical data and for each group by  column value already present in cache it add the data value to its             corresponding data cache value and outputs the row in case of a             incoming value having no match in index cache the new values for             group by and output ports are inserted into the cache .