1) "Why are the drapes open?"
2) Battattatatatta!
3) Michael goes down.

And I still don't know whether he saw something/somebody outside or he went down the very moment the first shot smashed the window.
What do you think?

My own theory:

Every night for the last ten years, Kay has gone to sleep leaving the drapes open. Michael comes in and wakes Kay, who says groggily, "Why are the drapes open?" Michael panics, and falls to the floor.

Seriously, I don't think Michael saw anything except the open drapes. I think he just had a flash of insight that things didn't add up at that moment.

Light travels immensely faster than sound or a bullet. He saw the gun's flash.

Logically, Michael was supposed to notice that the drapes were open, when they should have been closed. The gunmen were counting on him to meander over to the window to close the drapes--without looking outside. Michael's senses were alert and he looked out--and probably saw the gunmen aiming their weapons at him.

I replayed the sequence a few times, but didn't really come to a strong conclusion as to whether Michael was reacting to something he saw or heard.

In replaying the scene, it seemed like "the shooters" were much more visible when I rewound than when going forward. Is that something that is well-known? I guess it could also be a reflection from inside the room. I have the DVD "Collection" from around five years ago.

The reflexes are probably the result of Michael's military training. When they shoot, you hit the deck.

Maybe it was his gut instinct. I agree that the military training would help in hitting the deck.

And a bullet travels immensely faster than a human body leaping across the bed.

What bed? What leap?

I was wondering the same thing.

I believe Michael's reaction was due to the drapes being open. I'm sure that, due ot his position, he ran his house rigidly, and that included "small" things like the drapes being closed at a certain time. I also feel that his hitting the deck was because he saw the gunmen; otherwise, he would have hit the deck sooner - like immediatley upon seeing the open drapes.