If you think of the mechanics of a hook, you would be throwing the shot straight between your oponents guard so even if he has his hands up, chances are, he wont have them tight enough to block the hook. Potentially he could still block a straight shot.
As for throwing a straight after opening his stance, you've got similar reasons, however if you think, you should be throwing the straight as soon as your foot hits the floor after your kick so you are using your own momentum to power that straight down the middle.
does that make sense?
Perfect sense because his guard is likely open down the middle and the drop in weight (lead foot to floor) will add to the power of the rear cross.
Thanks for the lesson, its better teaching than my instructor gives lol