Well, it doesn't let you do anything. Like it won't let you move your mouse, and it won't even let you type anything. The only way to unblock it is to press ctrl, alt, del. Doesn't matter what's open, the only way is ctrl, alt, del.
If you want it to send keystrokes or mouse events while input is blocked you can use SendInput();
I made one a long time ago that blocked input then while the user couldn't control anything, the mouse opened up notepad.exe then it typed "I gotcha". It was so hilarious, but i can't find it though.