Originally Posted by Joe Lerch
Stretching from behind the razor when cutting against the grain can cause trouble. Since the whiskers are leaning towards the razor, the stretch is pulling the whiskers down, against the skin. So, if you're able to get under them, the strectch is pulling against the razor. On the other hand, if you're pulling from in front of the razor, you're making the whiskers stand up and you will be able to cut them more easily, at the risk of injuring your finger.
When stretching you always need to think about the direction of the stretch in relation to whether you're making the whiskers stand up or lie down.