Originally Posted by TheCheese
The last thing you should be thinking about is whether your arm is straight or not. It's just something that happens as a result of the technique. Federer and Nadal will commonly hit forehands without a straight arm if they're slightly out of position.
Not if you want to learn how to hit the straight-arm fh, as the OP does.
A number of top players have proven the technique, so it isn't correct to dismiss it out of hand to someone who wants to hit that way. A lot of what pros do is now subconscious, but it wasn't at a point when they were learning. Tennis isn't exactly natural to anyone.
I suggest that anyone wanting to learn the technique model it on a pro. You should shadow swing the technique in front of a mirror or camera to make sure you're hitting the required points. After a couple thousand repetitions you might not have to think about it on court.