Use these if you'd like to run a charger (i.e. the Sinewave Revolution) or an additional light in parallel with your Son Dynamo powered light.
The piggyback connector (pictured on the left) attaches to the SON hub the same way the standard 4.8mm Female Connector would (pcitured on the right), but it also provides an additional male tab to which an additional female connector is attached.
For connecting, you need a pair of these. This listing is for a single piggyback connector!