A Directional Transmission is a device that splits a single rotational input into two outputs based on the direction of the input. This can be useful if you wish to power two separate functions from a single motor. Of course, this additional flexibility doesn't come for free...the resulting outputs may only run in one direction.
There are lots of ways of building a directional transmission - many of which rely on a ratchet mechanism of some sort. The design presented here is quite different - it exploits the fact that a LEGO worm gear can slide freely on its axle. The resulting design is quite compact.
The 1x4 plates added in step 6 are only used to hold the two sides of the mechanism together. When incorporating this design into another model, they can usually be omitted.
The #8 axle added in step 3 is the input. The #6 axles in step 4 are the outputs. In order for the device to work properly there needs to be some resistance (load) on the output axles, otherwise the worm gear will not slide back and forth along the input axle as desired. In most cases, the output axles need to do some real "work" so this is not an issue.