I haven't yet listened to Lyoto Machida's post-fight interview, but I did see a video clip of his front kick knockout of Vitor Belfort on Saturday, May 12.
So I don't know if this is what Machida ("The Dragon") saw, but what I saw was Vitor rhythmically moving his guard up and down, up and down, at a regular pace. And Lyoto timed the kick to arrive on "down" when the head wasn't protected.
Fighters move their guard like that all the time. Keeping the guard moving can confuse the opponent, it expands the overall area covered by the guard, and it keeps the hands active and ready to respond. All good things.
Unfortunately, if the movement is regular, and thus predictable, and you are also in your opponent's range - well, this is what happened.
The video was posted by Vitor himself, alongside an older video of Anderson Silva ("The Spider") famously knocking him out by front-kick. I guess Vitor has a healthy sense of absurdity, and respect to him for that.
But the Silva kick seems different to me. Vitor wasn't moving his hands up and down, up and down, but rather holding them in front of his face, which you would think would be a good way to protect the head.
But Silva's kick goes right between the two hands.
No guard is perfect, so what I take away from this is to stay just out of range unless one is attacking. And when in range while attacking, be as erratic - unpredictable - as possible.