SendInput operates at the bottom level of the input stack. It is just a backdoor into the same input mechanism that the keyboard and mouse drivers use to tell the window manager that the user has generated input. The SendInput function doesn't know what will happen to the input. That is handled by much higher levels of the window manager, like the components which hit-test mouse input to see which window the message should initially be delivered to. So if your goal is to change the way you call SendInput so it changes the focus management rules, you're barking up the wrong tree. It's like asking, "Please tell me how RAM chips work so I can use it to change the way Lotus 1-2-3 resolves circular references."
Read more: The old new thing