Dynamic Runner does a quick tutorial on how to tightening locs with a latch hook. The process is called interlocking.
The process of interlocking locs requires a tool appropriately called an interlocking tool. It is made from a durable, flexible plastic. There are some metal latch-hook tools used for crocheting that some recommend instead. But they should be avoided because they snag and pull hairs from the scalp causing significant damage. (A proper interlocking tool for the hair can be found at www.drkariwilliams.com/store)
More information on Starting Locs.