The Hot Key trigger is perhaps the most common and most basic of all triggers. When you press the configured keyboard key, the system swallows the keystroke, and Keyboard Maestro executes the macro.
The key can be a letter, number, symbol or function key, often in combination with one or more modifiers (Shift, Control, Option and/or Command). Almost any key can be a trigger, and keep in mind the number pad counts as different keys to the numeric keys on the main keyboard.
You can type the desired key or key combination in the hot key box, or select a predefined key (holding down any desired modifiers) from the popup menu to its right. Note that there is a relatively prevalent third party / system bug that makes the system think it is permanently in a password field, and this will prevent entering a hot key by typing in this manner.
The keystroke may include zero or more of the normal modifiers (Command, Option, Shift, Control), together with another key (such as a letter, number, function key or other keys). The Fn key is not a modifier and cannot be used as such. It retains its normal purpose in toggling the function keys role, and so it might be part of a hot key, but only if the function keys normally have their hardware (eg brightness up) role and then Fn key makes them into a function key suitable for use in a hot key trigger.
You can configure the hot key trigger to execute the macro when the key is pressed, when it is released, or repeatedly while the key is held down. This allows you to do things like have a macro execute when the key is pressed, and then a second macro execute when the key is released, for example to toggle a setting on and then off again.
You can configure the hot key trigger to execute when the key is tapped (press and released quickly) each time, the first tap, double tapped, triple tapped, or quadruple tapped (v7.0+). Keep in mind that Keyboard Maestro cannot see the future, so “tapped once” will fire even on the first tap even if you tap the key twice (“tapped” would fire both times).
Hot Keys will override application menu Command Keys or any other keys typically used in applications, but if any other application registers the same hot key, then both your macro and the other application's action will happen.
⚠️ Note that whether or not a key typed by Keyboard Maestro itself will trigger a hot key (and thus be swallowed and execute a macro) or not (and thus go through to the application is not defined - either behaviour may happen and which behaviour happens will vary depending on many unpredictable factors.
So keep in mind that regardless of whether you trigger the macro on hot key pressed, down, released or tapped, the key is swallowed by the system and cannot be used for its normal purposes. You can use the USB_Device_Key trigger if you you want to trigger the macro without the system swallowing the key.
Modifiers alone (eg just Control) cannot be used as a hot key, although you can use the USB_Device_Key trigger to detect modifier key presses as well as other button presses like the mouse buttons or keys on non-standard keyboards like the XK-24.
Hot Keys suffer from the drawback that you need to remember a cryptic keystroke. This can be mitigated by selecting consistent keystrokes (such as Control-Letter to mean insert text and Control-Option-Letter to mean launch an application). You can also use a tool like KeyCue to display command keys and macro hot keys.
To further help with this, if multiple macros are executed with the same hot key, the conflicting macros are displayed in a palette allowing you to select the desired macro. You can select a macro from the palette using either number keys, or by typing the first distinct character to filter the macros down until only one is left. You could use this feature to allow a single hot key to do multiple user-selected actions.
A note on terminology: