Double Terminated crystals have a termination at each end, thus the energy can flow in either direction and can be guided to do so via thought patterns.
They are also very useful to facilitate balance within any given energy fields. An example of this is where a root chakra may be overactive and the sacral chakra underactive. Place a Double Terminated crystal between them and the two chakras will self balance (in most cases).
These crystals are very exciting to hold between the palms of your hands and feel their energy running through and around your body.
Double Terminated crystals are also very useful tools for crystal grids and mandalas.