Its most common use is to merge data flows before feeding them to a multiconnector element (which cannot merge flows itself).
A Switcher is intended to direct all the data in an incoming path onto a selection from two output paths. It may be sent to one or other path, to both, or to neither. 'Director' messages sent to its 'Control' input are used to determine this. (It can also be set directly from its control panel.)
You'll notice that a Switcher element is unique (at the moment) in having three output pips. Two of these are the possible output connections ('Through' and 'Diverted'); the third is simply a pass-through of the 'Control' input if this should be wanted. There are two inputs: the incoming data (labelled 'Through', to match the undiverted output connector), and 'Control', where director messages will arrive.
'Director' messages are distinct from all other data flowing around a diagram, though they can use the same paths if it's convenient (usually probably not...). They are generated only by modules specifically designed to do so and are recognized only by the Switcher. Other elements simply ignore them. Of course if you send Director data to the 'Through' input, it will be handled like anything else, and switched to the current output(s).
Director events, as handled by the Switcher, have sixteen possible 'tags' that can be set. Any subset of these tags may appear in a single event (depending on the source), and a Switcher will respond to them (or not) according to the checkboxes set in its panel. (There is an alternative type of director message that has a single numeric value code, rather than a set of tags. The Switcher, however, accepts the first sixteen of these codes as tags.)
At the top of the Switcher's control panel is an Indicator/Selector that shows the current state; clicking on this will bring up a menu allowing you to set the state manually. The possible states are Pass, Divert, Dupe, and Stop. In Pass, all data just goes straight on out the 'Through' output. With Divert, the data only appears on the 'Diverted' output; Dupe sends it to both, and Stop blocks it altogether.
The rest of the panel is taken up with the array of checkboxes that determine its response to director messages. A marked checkbox in a column will cause a matching tag in an incoming director message to set the switcher to the corresponding state (which will be reflected in the indicator above). You can check as many boxes as you wish in a column, but only one in each row. (If a message should have more than one tag set, and these happen to match conflicting checkbox settings, columns to the right take priority.)
Be careful that you connect to the intended inputs and outputs. The default (first connected) input and output are for data ('Through'). Use the popup menu on the connector after the link is made to ensure you are connected as you think.
A director message needs at least one of its tags set to have any effect on a Switcher. The TriggerSwitch control panel has a set of checkboxes to select the ones you want. It also has a button to trigger a message directly if you wish.
The element has only one input ('Trigger'). The outputs are 'Switch Out', where the director messages appear, and a pass-through of 'Trigger'.