Does your heart leap, does it sink, do you get butterflies in your stomach, in the moment when you first see a message from that person? That's how you really feel, and if you didn't think you felt that way, you thought wrong. Schopenhauer's trick relies on the fact that emotion is faster than thought. A letter takes you by surprise: even if you're expecting to hear from someone, you don't know exactly when it will arrive. It arrives: in that first second your emotions have a chance to show through, before your thoughts have got into gear. It works with emails and phone calls as well, of course, but not with any encounter which is planned out in advance. The point is that you do not enjoy direct and perfect knowledge of your own feelings. You can be wrong about them, just like you could misjudge anyone else's feelings. Maybe you think that you like someone, when you really find them annoying. You believe that you like someone as a friend, but you really feel more than that. In fact, it's not clear that we have any special insight into our own emotions, beyond that which is available to others. We tend to assume that we do. For one thing, we say they're our emotions: we own them. I'm the one who feels my emotions, and emotions are just feelings, so I must be the expert on them, right? Yes, but feeling an emotion and understanding it are entirely separate. As I wrote previously, we all interpret our feelings in various ways, and like any act of interpretation, we can be either right or wrong. Suppose I love you and I think "I love you". In that case I'm right. But I could love you and think I don't (maybe I think it's just lust), or then again I could not love you (it is just lust) but think that I do. Any combination of feelings and thoughts is possible. The notion that our mind is a single monolithic thing, and that we know everything that's in our own mind, is a stubborn one, but quite misleading. In fact we know very little about what goes on in our own heads; 100 billion cells are firing all the time, and we're not aware of any of them. Sometimes we can achieve self-knowledge, but it is never guaranteed.
If you want to find out your real opinion of anyone, observe the impression made upon you by the first sight of a letter from him.