$

This is usually only necessary if the script uses the Send command to send the keys that comprise the hotkey itself, which might otherwise cause it to trigger itself. The $ prefix forces the keyboard hook to be used to implement this hotkey, which as a side-effect prevents the Send command from triggering it. The $ prefix is equivalent to having specified #UseHook somewhere above the definition of this hotkey.

The $ prefix has no effect for mouse hotkeys, since they always use the mouse hook. It also has no effect for hotkeys which already require the keyboard hook, including any keyboard hotkeys with the tilde (~) or wildcard (*) modifiers, key-up hotkeys and custom combinations.

[v1.1.06+]: #InputLevel and SendLevel provide additional control over which hotkeys and hotstrings are triggered by the Send command.

https://autohotkey.com/docs/Hotkeys.htm

space::
sendinput, {space 2}