This action directly sends a command to a UI element. What UI actions are available depends on the UI element. When you pick one, it will show you a list of UI actions to choose from. Some examples are that windows typically have a "raise" action, Finder icons have a "show menu" action, scroll bars have a "increase value" and "decrease value" action, and so forth.
In this example a 'Press' action is to be performed on the UI Element.
Essentially this Flexi-deck action is another way of interacting with UI elements that may sometimes work better than clicking them, depending on the task at hand.
One advantage of the "Send command" action over a click is that it doesn't require the UI element being fully visible on screen (as you are issuing the command directly to the UI element rather than indirectly through a click), it does also have some disadvantages too in that in some instances it does not work as expected as per the note below!
Note: Use this command with caution - if it works its very useful, less so in cases described below!
The method provided by Apple that Flexi-deck uses to send a command to a UI element does not always work as intended, and does not necessarily work in a manner that will be useful for a macro. Some UI elements will fail to respond to command without issuing an error (i.e. "silently fail") even though they report being able to receive the command. In other cases, the command will not finish until there is some kind of user interaction. For example, there are buttons that can receive a command to open a pop-up menu, but the command does not finish executing until the pop-up menu has been dismissed by the user, causing the macro to pause as long as the pop-up menu is open.
If the "Send Command to UI Element" does not seem to have any effect, causes errors in the execution of the macro, or causes long, unintended pauses, try using one of the other actions to produce the effect you want, such as "Click" or "Perform Keystroke".